U.S. patent number 5,208,812 [Application Number 07/573,308] was granted by the patent office on 1993-05-04 for telecommunications system.
This patent grant is currently assigned to British Telecommunications Public Limited Company, Ferranti Creditphone Limited, GEC Plessey Telecommunications Limited, Mercury Communications Limited, Orbitel Mobile Communications Limited, Phonepoint Limited, Shaye Communications Limited, STC PLC. Invention is credited to Michael T. Dudek, Rupert Goodings, Emlyn Jones, David C. Odhams, Peter N. Proctor.
United States Patent |
5,208,812 |
Dudek , et al. |
May 4, 1993 |
Telecommunications system
Abstract
A communication procedure suitable for a cordless telephone
system involves time division duplex radio communication between a
handset 11 and a base station 3 using alternating bursts of
transmission over a single radio channel. Once a radio link has
been set up, initial transmissions carry a synchronization logical
channel S and a signalling logical channel D multiple together, bu
the link may switch to bursts carrying a communications logical
channel B for the speech data and the signalling logical channel D.
Bursts synchronization is achieved by the asynchronous detection of
words in a synchronization channel S. These words have bit patterns
reducing the probability of incorrect asynchronous detection
thereof. If one part ceases to receive handset signals from the
other, it transmits a special signal to inform the other part. This
enables both parts to detect the failure of a link at substantially
the same time, so that their actions to reestablish the link are
synchronized.
Inventors: |
Dudek; Michael T. (Agoura,
CA), Goodings; Rupert (Cambridge, GB3), Jones;
Emlyn (Harlow, GB3), Odhams; David C. (Epping,
GB3), Proctor; Peter N. (Basingstoke,
GB3) |
Assignee: |
British Telecommunications Public
Limited Company (London, GB)
Ferranti Creditphone Limited (Moston, GB)
GEC Plessey Telecommunications Limited (Coventry,
GB)
Mercury Communications Limited (London, GB)
Orbitel Mobile Communications Limited (Basingstoke,
GB)
Shaye Communications Limited (Winchester, GB)
Phonepoint Limited (London, GB)
STC PLC (London, GB)
|
Family
ID: |
26294881 |
Appl.
No.: |
07/573,308 |
Filed: |
November 26, 1990 |
PCT
Filed: |
January 26, 1990 |
PCT No.: |
PCT/GB90/00120 |
371
Date: |
November 26, 1990 |
102(e)
Date: |
November 26, 1990 |
PCT
Pub. No.: |
WO90/09073 |
PCT
Pub. Date: |
August 09, 1990 |
Foreign Application Priority Data
|
|
|
|
|
Jan 27, 1989 [GB] |
|
|
8901756 |
Jan 27, 1989 [GB] |
|
|
8901823 |
|
Current U.S.
Class: |
370/280; 375/368;
370/350; 370/524; 370/513 |
Current CPC
Class: |
H04M
1/733 (20130101); H04J 3/0602 (20130101); H04J
3/0652 (20130101); H04M 1/72505 (20130101); H04L
7/041 (20130101) |
Current International
Class: |
H04J
3/06 (20060101); H04M 1/72 (20060101); H04M
1/725 (20060101); H04M 1/733 (20060101); H04L
7/04 (20060101); H04J 003/06 (); H04L 007/00 () |
Field of
Search: |
;370/29,105.4,100.1,29
;375/116 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Kuntz; Curtis
Assistant Examiner: Ghebrettinsae; T.
Attorney, Agent or Firm: Frishauf, Holtz, Goodman &
Woodward
Claims
We claim:
1. A telecommunication system comprising:
a device of a first type and a plurality of devices of a second
type, said device of the first type comprising communication means
for conducting time-division two-way communication with any one of
said devices of the second type over a radio channel by exchanging
radio signals in alternating bursts carrying digital data, such
that during the time-division two-way communication, transmission
of one of said alternating bursts from one of the devices is
completed before transmission of the next of said alternating
bursts by the other of the devices is begun, and each of said
devices of the second type comprising communication means for
conducting said time-division two-way communication with the device
of the first type;
the device of the first type comprising means for providing said
bursts such that at least some of the bursts contain a first
synchronisation pattern or one of a group of first synchronization
patterns, and detecting means for asynchronously detecting a second
synchronisation pattern or one of a group of second synchronisation
patterns in a received burst to enable it to determine timing
information about said received burst, each device of the second
type comprising means for producing said bursts such that at least
some of the bursts contain said second synchronisation pattern or
one of said group of second synchronisation patterns, and detecting
means for asynchronously detecting said first synchronisation
pattern for one of said group of first synchronisation patterns in
a received burst to enable it to determine timing information about
said received burst, said first synchronization pattern or patterns
being different from said second synchronization pattern or
patterns, said detecting means in the plurality of devices of the
second type not responding to reception of the second
synchronisation pattern or patterns, whereby each of said plurality
of devices of the second type does not respond to reception of
transmissions or any other device of the second type.
2. A method of telecommunication comprising the steps of:
establishing time-division two-way communication over a radio
channel between a device of a first type, acting at a given time as
one of a receiver and a transmitter, and any one of a plurality of
devices of a second type, acting at said given time as the other of
said receiver and transmitter, by exchanging radio signals in
alternating bursts carrying digital data, such that during the
time-division two-way communication, transmission of one of said
alternating bursts from one of the devices is completed before
transmission of the next of said alternating bursts by the other of
the device is begun;
producing said bursts such that at least some of the bursts contain
a synchronisation pattern;
asynchronously detecting said synchronisation pattern by the
receiver to enable it to determine timing information about the
received bursts, the device of the first type transmitting a first
synchronisation pattern or one of a group of first synchronisation
patterns and the devices of the second type transmitting a second
synchronisation pattern or one of a group of second synchronisation
patterns, the first synchronisation pattern or patterns being
different from the second synchronisation pattern or patterns;
and
preventing the plurality of devices of the second type from
responding to reception of the second synchronisation pattern or
patterns, whereby each of said plurality of devices of the second
type does not respond to reception of transmissions by any other
device of the second type.
3. A system according to claim 1 wherein said time-division two-way
communication may be performed by any one of a plurality of devices
of the first type, and detecting means in the plurality of devices
of the first type do not respond to reception of the first
synchronisation pattern or patterns, whereby each of the plurality
of devices of the first type does not respond to transmissions by
any other device of the first type.
4. A system according to claim 1 or claim 3 wherein the burst
producing means of each said device of one type transmits a
predetermined synchronisation pattern while attempting to initiate
communication by said radio signals with a device of the other
type, and subsequently transmits a different predetermined
synchronisation pattern after said communication has been
initiated.
5. A telecommunication system comprising:
first and second devices having communication means for providing
time-division two-way communication between said devices over a
radio channel by exchanging radio signals in alternating bursts
carrying digital data, such that during the time-division two-way
communication, transmission of one of said alternating bursts from
one of the first and second devices is completed before
transmission of the next burst by the other of the first and second
devices is begun;
means for producing bursts such that at least some of the bursts
transmitted by a said device are bursts of a particular format and
contain a synchronisation pattern of L bits of data and also bits
of variable data;
means for asynchronously detecting said synchronisation pattern by
the receiver to enable it to determined the timing information
about the transmitted bursts of said particular format;
means in the receiving device which determines the synchronisation
pattern to be present in the received data when a comparison
operation between the received data and a stored copy of the
synchronisation pattern results in no more than K bits of the
received data failing the comparison, where K is zero or a positive
integer; and
the arrangement of bits in each said burst of said particular
format being such that in any consecutive string of L bits of data
in the burst, there are less than L-K bits of variable data.
6. A method of telecommunication in which first and second devices
communicate with each other, comprising the steps of:
performing time-division two-way communication between said devices
over a radio channel by exchanging radio signals in alternating
bursts carrying digital data, such that during the time-division
two-way communication, transmission of one of said alternating
bursts from one of the first and second devices is completed before
transmission of the next of said alternating bursts by the other of
the first and second devices is begun;
producing said bursts such that at least bursts of a particular
format transmitted by a said device contain a synchronisation
pattern of L bits of data and also bits of variable data;
asynchronously detecting said synchronisation pattern by the
receiver to enable it to determine timing information about the
transmitted bursts of said particular format;
determining in the receiver that the synchronisation pattern is
present in the received data when a comparison operation between
the received data and a stored copy of the synchronisation pattern
results in no more than K bits of the received data failing the
comparison, where K is zero or a positive integer; and
arranging the bits in each said burst of said particular format
such that in any consecutive string of L bits of data in the burst
of said particular format, there are less than L-K bits of variable
data.
7. A system according to claim 5 wherein the arrangement of bits in
each said burst of said particular format is such that in any
consecutive string of L bits of data in the burst of said
particular format, there are no more than L-K-6 bits of variable
data.
8. A telecommunication system comprising:
first and second devices having communication means for providing
time-division two-way communication between said devices over a
radio channel by exchanging radio signals in alternating bursts
carrying digital data, such that during the time-division two-way
communication, transmission of one of said alternating bursts from
one of the first and second devices is completed before
transmission of the next of said alternating bursts by the other of
the first and second devices is begun;
means for producing bursts including alternating bursts, such that
at least some of the bursts produced by said means for producing
bursts are bursts of a particular format of digital data and
comprise a first portion having a repeating pattern of fixed value
and variable bits and a second portion comprising an L-bit
synchronisation pattern;
means for asynchronously detecting said synchronisation pattern by
the receiving device to enable it to determine timing information
about the bursts of said particular format; and
means in the receiving device which determines the synchronisation
pattern to be present in the received data when a comparison
operation between the received data and a stored copy of the
synchronisation pattern results in no more than K bits of the
received data failing the comparison, where K is zero or a positive
integer;
the L-bit synchronisation pattern and the repeating pattern of
fixed value and variable bits being such that a string of L
successive bits of the repeating pattern, starting at any position
in a repeat of the pattern, matches less than L-K bits of the
synchronisation pattern even if it is assumed that every variable
bit in the string provides a match.
9. A method of telecommunication in which first and second devices
communicate with each other, comprising the steps of:
performing time-division two-way communication between said devices
over a radio channel by exchanging radio signals in alternating
bursts carrying digital data, such that during the time-division
two-way communication, transmission of one of said alternating
bursts from one of the first and second devices is completed before
transmission of the next of said alternating bursts by the other of
the first and second devices is begun;
producing bursts such that at least bursts of a particular format
of digital data comprise a first portion having a repeating pattern
of fixed value and variable bits and a second portion comprising an
L-bit synchronisation pattern;
asynchronously detecting said synchronisation pattern by the
receiver to enable it to determine timing information about the
bursts of said particular format;
determining the synchronisation pattern to be present in the
received data when a comparison operation between the received data
and a stored copy of the synchronisation pattern results in no more
than K bits of the received data failing the comparison, where K is
zero or a positive integer; and
selecting the L-bit synchronisation pattern and the repeating
pattern of fixed value and variable bits such that a string of L
successive bits of the repeating pattern, starting at any position
in a repeat of the pattern, matches less than L-K bits of the
synchronisation pattern even if it is assumed that every variable
bit in the string provides a match.
10. A system according to claim 8, wherein any said string of L
successive bits of the said repeating pattern matches no more than
L-K-2 bits of the synchronisation pattern even if it is assumed
that every variable bit of the string provides a match.
11. A telecommunication system comprising:
first and second devices having communication means for providing
time-division two-way communication between said devices over a
radio channel by exchanging radio signals in alternating bursts
carrying digital data, such that during the time-division two-way
communication, transmission of one of said alternating bursts from
one of the first and second devices is completed before
transmission of the next of said alternating bursts by the other of
the first and second devices is begun;
means for producing bursts such that at least some are bursts of a
particular format of digital data and comprise an L-bit
synchronisation pattern;
means for asynchronously detecting said synchronisation pattern by
the receiving device to enable it to determine timing information
about the bursts of said particular format; and
means in the receiver to determine the synchronisation pattern to
be present in the received data when a comparison operation between
the received data and a stored copy of the synchronisation pattern
results in no more than K bits of the received data failing the
comparison, where K is zero or a positive integer;
the synchronisation pattern being adjacent a portion of the burst
made up of fixed value bits, and the number of matches between the
synchronisation pattern and any string of L successive bits of the
burst of said particular format composed only of at least a part of
said portion of fixed value bits and an adjacent part of the
synchronisation pattern being less than L-K.
12. A method of telecommunication in which first and second devices
communicate with each other, comprising the steps of:
performing time-division two-way communication between said devices
over a radio channel by exchanging radio signals in alternating
bursts carrying digital data, such that during the time-division
two-way communication, transmission of one of said alternating
bursts from one of the first and second devices is completed before
transmission of the next of said alternating bursts by the other of
the first and second devices is begun;
producing bursts such that at least bursts of a particular format
of digital data comprise an L-bit synchronisation pattern;
asynchronously detecting said synchronisation pattern by a receiver
to enable it to determine timing information about bursts of said
particular format;
determining the receiver that the synchronisation pattern is
present in the received data when a comparison operation between
the received data and a stored copy of the synchronisation pattern
results in no more than K bits of the received data failing the
comparison, where K is zero or a positive integer; and
arranging the burst of said particular format such that the
synchronisation pattern is adjacent a portion of the burst of said
particular format made up of fixed value bits, and the number
matches between the synchronisation pattern and any string of L
successive bits of the burst of said particular format composed
only of at least a part of the said portion of fixed value bits and
an adjacent part of the synchronisation pattern being less than
L-K.
13. A system according to claim 11, wherein said burst producing
means produces a preset synchronisation pattern from among a
predetermined plurality of L-bit synchronisation patterns, and the
number of matches between anyone of said plurality of
synchronisation patterns and any string of L successive bits of the
burst composed only of any other of said plurality of
synchronisation patterns or at least a part of the said portion of
fixed value bits and an adjacent part of any other said
synchronisation pattern is less than L-K.
14. A system according to claim 13 wherein, for at least some of
the said plurality of synchronisation patterns, the said number of
matches does not exceed L-K-8.
15. A system according to claim 13, wherein, for all of the said
plurality of synchronisation patterns, the said number of matches
does not exceed L-K-7.
16. A system according to any one of claims 5, 7, 8, 10, 11, and 13
to 15 in which K is not zero.
17. A system according to claim 16 in which K is two.
18. A telecommunication system comprising:
first and second devices having communication means for providing
time-division two-way communication between said devices over a
radio channel by exchanging radio signals in alternating bursts
carrying digital data, such that during the time-division two-way
communication, transmission of one of said alternating bursts from
one of the first and second devices is completed before
transmission of the next of said alternating bursts by the other of
the first and second devices is begun;
means for producing bursts such that at least some are bursts of a
particular format of digital data and comprise an L-bit
synchronisation pattern;
means for asynchronously detecting said synchronisation pattern by
a receiver to enable it to determine timing information about the
bursts of said particular format;
the synchronisation pattern having a peak self-correlation side
lobe value of not more than +2, for any amount of offset, where the
self-correlation side lobe value at an amount of offset is defined
as the number of matches between bits of the pattern and itself
offset by the amount, minus the number of mismatches between the
bits of the pattern and itself at the same amount of offset.
19. A method of telecommunication in which first and second devices
communicate with each other, comprising the steps of:
performing time-division two-way communication between said devices
over a radio channel by exchanging radio signals in alternating
bursts carrying digital data, such that during the time-division
two-way communication, transmission of one of said alternating
bursts from one of the first and second devices is completed before
transmission of the next of said alternating bursts by the other of
the first and second devices is begun;
producing bursts such that at least bursts of a particular format
of digital data comprise an L-bit synchronisation pattern;
asynchronously detecting said synchronisation pattern by a receiver
to enable it to determine timing information about the bursts of
said particular format; and
arranging the synchronisation pattern to have a peak
self-correlation side lobe value of not more than +2, for any
amount of offset, where the self-correlation side lobe value at an
amount of offset is defined as the number of matches between bits
of the pattern and itself offset by the amount, minus the number of
mismatches between the bits of the pattern and itself at the same
amount of offset.
20. A telecommunication system comprising:
first and second devices having communication means for providing
time-division two-way communication between said devices over a
radio channel by exchanging radio signals in alternating bursts
carrying digital data, such that during the time-division two-way
communication, transmission of one of said alternating bursts from
one of the first and second devices is completed before
transmission of the next of said alternating bursts by the other of
the first and second devices is begun;
means for producing bursts such that at least some are bursts of a
particular format of digital data and comprise a 24-bit
synchronisation pattern which, when given in hexadecimal format, is
one of: BE4E50; 41B1AF; EB1B05; 14E4FA; 0A727D; F58D82; A0D8D7; and
5F2728;
means for asynchronously detecting said synchronisation pattern by
a receiver to enable it to determine timing information about the
bursts of said particular format.
21. A method of telecommunication in which first and second devices
communicate with each other, comprising the steps of:
performing time-division two-way communication between said devices
over a radio channel by exchanging radio signals in alternating
bursts carrying digital data, such that during the time-division
two-way communication, transmission of one of said alternating
bursts from one of the first and second devices is completed before
transmission of the next of said alternating bursts by the other of
the first and second devices is begun;
producing bursts such that at least bursts of a particular format
of digital data and comprise a 24-bit synchronisation pattern when,
given in hexadecimal format, is one of: BE4E50; 41B1AF; EB1B05;
14E4FA; 0A727D; F58D82; A0D8D7; and 5F2728;
asynchronously detecting said synchronisation pattern by a
receiving device to enable it to determine timing information about
the bursts of said particular format.
22. A method according to claim 2, wherein said time-division
two-way communication may be performed by any one of a plurality of
devices of the first type, and the plurality of devices of the
first type do not respond to reception of the first synchronisation
pattern or patterns, whereby each of the plurality of devices of
the first type does not respond to transmissions by any other
device of the first type.
23. A method according to claim 2 or claim 22, wherein the step of
providing said bursts of a particular format includes a said device
of one type transmitting a predetermined synchronisation pattern
while attempting to initiating communication by said radio signals
with a device of the other type, and subsequently transmitting a
different predetermined synchronisation pattern after said
communication has been initiated.
24. A method according to claim 6, wherein the arrangement of bits
in each said burst of said particular format is such that in any
consecutive string of L bits of data in the burst of said
particular format, there are no more than L-K-6 bits of variable
data.
25. A method according to claim 9, wherein any said string of L
successive bits of the said repeating pattern matches no more than
L-K-2 bits of the synchronisation pattern even if it is assumed
that every variable bit of the string provides a match.
26. A method according to claim 12, wherein said step of producing
bursts of a particular format comprises producing a preset
synchronisation pattern from among a predefined plurality of L-bit
synchronisation patterns, and the number of matches between any of
said plurality of synchronisation patterns and any string of L
successive bits of the particular burst composed only of any other
said synchronisation pattern or at least a part of the said portion
of fixed value bits and an adjacent part of any other said
synchronisation pattern is less than L-K.
27. A method according to claim 26, wherein, for at least some of
the said plurality of synchronisation patterns, the said number of
matches does not exceed L-K-8.
28. A method according to claim 26, wherein, for all of said
plurality of synchronisation patterns, the said number of matches
does not exceed L-K-7.
29. A method according to any one of claims 6, 9, 12, 24, 25, 26,
27 and 28, wherein K is not zero.
30. A method according to any one of claims 6, 9, 12, 24, 25, 26,
27 and 28, wherein K is two.
31. A telecommunication device for establishing time-division
two-way communication with another device over a radio channel by
exchanging radio signals in alternating bursts carrying digital
data such that, during the time-division two-way communication,
reception of a said burst from said other device is completed
before transmission of the next burst by said telecommunication
device is begun;
said telecommunication device comprising:
detecting means for asynchronously detecting a first
synchronisation pattern or one of a group of first synchronisation
patterns in a received burst to enable the telecommunication device
to determine timing information about said received burst; and
means for producing said bursts such that at least some of the
bursts contain a second synchronisation pattern or one of a group
of second synchronisation patterns;
said first synchronisation pattern or patterns being different from
said second synchronisation pattern or patterns and said detecting
means not responding to said second synchronisation pattern or
patterns.
32. A telecommunication device for establishing time-division
two-way communication with another device over a radio channel by
exchanging radio signals in alternating bursts carrying digital
data such that, during the time-division two-way communication,
reception of a said burst from said other device is completed
before transmission of the next burst by said telecommunication
device is begun;
said telecommunication device comprising:
means for producing bursts for transmission by the
telecommunication device at least some of which are bursts of a
type containing bits of variable data and a synchronisation pattern
of L bits of data, the arrangement of bits in bursts of said type
being such that any consecutive string of L bits of data in the
burst contains less than L-K consecutive bits of variable data,
where K is a predetermined value which is zero or a positive
integer.
33. A telecommunication device for establishing time-division
two-way communication with another device over a radio channel by
exchanging radio signals in alternating bursts carrying digital
data such that, during the time-division two-way communication,
reception of a said burst from said other device is completed
before transmission of the next burst by said telecommunication
device is begun;
said telecommunication device comprising:
detecting means for asynchronously detecting a synchronisation
pattern of L bits of data in a received burst to enable the
telecommunication device to determine timing information about the
burst; and
decoding means for decoding at least some received bursts according
to a predetermined burst format;
said detecting means determining that said synchronisation pattern
is present in received data when a comparison operation between the
received data and a stored copy of the synchronisation pattern
results in no more than K bits of the received data failing the
comparison, where K is zero or a positive integer; and
said predetermined format being such that a burst in said format
contains bits of variable data and said synchronisation pattern,
and in any consecutive string of L bits of data in a burst in said
format there are less than L-K consecutive bits of variable
data.
34. A telecommunication device for establishing time-division
two-way communication with another device over a radio channel by
exchanging radio signals in alternating bursts carrying digital
data such that, during the time-division two-way communication,
reception of a said burst from said other device is completed
before transmission of the next burst by said telecommunication
device is begun;
said telecommunication device comprising:
means for producing bursts for transmission by the
telecommunication device including said alternating burst such that
at least some of the bursts produced by said means for producing
bursts are bursts of a particular format of digital data such that
a burst in said particular format comprises a first portion having
a repeating pattern of fixed value bits and variable bits and a
second portion comprising an L bit synchronisation pattern, the L
bit synchronisation pattern and the repeating pattern of fixed
value bits and variable bits being such that a string of L
successive bits of the repeating pattern, starting at any position
in a repeat of the pattern, matches less than L-K bits of the
synchronisation pattern even if it is assumed that every variable
bit in the string provides a match, where K is a predetermined
value which is zero or a positive integer.
35. A telecommunication device for establishing synchronous
time-division two-way communication with another device over a
radio channel by exchanging radio signals with said other device in
alternating bursts carrying digital data such that, during the
time-division two-way communication, reception of a said burst from
said other device is completed before transmission of the next
burst by said telecommunication device is begun;
said telecommunication device comprising:
detecting means for asynchronously detecting an L bit
synchronisation pattern in a received burst to determine timing
information about the burst, said detecting means determining that
said synchronisation pattern is present in received data when a
comparison operation between the received data and a stored copy of
the synchronisation pattern results in a no more than K bits of the
received data failing the comparison, where K is zero or a positive
integer; and
decoding means for decoding at least some received bursts according
to a particular format of digital data such that a burst in said
particular format comprises a first portion having a repeating
pattern of fixed value bits and variable bits and a second portion
comprising said L bit synchronisation pattern;
the L bit synchronisation pattern and the repeating pattern of
fixed value bits and variable bits being such that a string of L
successive bits of the repeating pattern, starting at any position
in a repeat of said pattern, matches less than L-K bits of the
synchronisation pattern even if it is assumed that every variable
bit in the string provides a match.
36. A telecommunication device for establishing time-division
two-way communication with another device over a radio channel by
exchanging radio signals in alternating bursts carrying digital
data such that, during the time-division two-way communication,
reception of a said burst from said other device is completed
before transmission of the next burst by said telecommunication
device is begun;
said telecommunication device comprising:
means for producing bursts for transmission by the
telecommunication device at least some of which are bursts of a
particular format of digital data such that a burst in said
particular format comprises an L bit synchronisation pattern
adjacent a portion of the burst made up of fixed value bits, the L
bit synchronisation pattern and said fixed value bits being such
that the number of matches between the synchronisation pattern and
any string of L successive bits of a burst in said particular
format which string is composed only of at least a part of said
portion of fixed value bits and an adjacent part of the
synchronisation pattern is less than L-K, where K is a
predetermined value which is zero or a positive integer.
37. A telecommunication device for establishing time-division
two-way communication with another device over a radio channel by
exchanging radio signals in alternating bursts carrying digital
data such that, during the time-division two-way communication,
reception of a said burst from said other device is completed
before transmission of the next burst by said telecommunication
device is begun;
said telecommunication device comprising:
detecting means for asynchronously detecting an L bit
synchronisation pattern in a received burst to determine timing
information about the burst, said detecting means determining that
said synchronisation pattern is present in received data when a
comparison operation between the received data and a stored copy of
the synchronisation pattern results in a no more than K bits of the
received data failing the comparison, where K is zero or a positive
integer; and
decoding means for decoding at least some received bursts according
to a particular format of digital data such that a burst in said
particular format comprises an L bit synchronisation pattern
adjacent a portion of the burst made up of fixed value bits;
the L bit synchronisation pattern and said fixed value bits being
such that the number of matches between the synchronisation pattern
and any string of L successive bits of a burst in said particular
format which string is composed only of at least a part of said
portion of fixed value bits and an adjacent part of the
synchronisation pattern is less than L-K.
38. A telecommunication device for establishing time-division
two-way communication with another device over a radio channel by
exchanging radio signals in alternating bursts carrying digital
data such that, during the time-division two-way communication,
reception of a said burst from said other device is completed
before transmission of the next burst by said telecommunication
device is begun;
said telecommunication device comprising:
means for producing bursts for transmission by the
telecommunication device at least some of which are bursts of a
particular format of digital data such that a burst in said
particular format comprises an L bit synchronisation pattern having
a peak self-correlation side lobe value of not more than +2 for any
amount of offset, where the self-correlation side lobe value at an
amount of offset is defined as a number of matches between bits of
the pattern and itself offset by the amount, minus the number of
mismatches between bits of the pattern and itself at the same
amount of offset.
39. A telecommunication device for establishing time-division
two-way communication with another device over a radio channel by
exchanging radio signals in alternating bursts carrying digital
data such that, during the time-division two-way communication,
reception of a said burst from said other device is completed
before transmission of the next burst by said telecommunication
device is begun;
said telecommunication device comprising:
means for asynchronously detecting a predetermined synchronisation
pattern in a received burst to determine timing information about
the burst, the predetermined synchronisation pattern being such
that it has a peak self-correlation side lobe value of not more
than +2 for any amount of offset, where the self-correlation side
lobe value at an amount of offset is defined as a number of matches
between bits of the pattern and itself offset by the amount, minus
the number of mismatches between bits of the pattern and itself at
the same amount of offset.
40. A telecommunication device for establishing time-division
two-way communication with another device over a radio channel by
exchanging radio signals in alternating bursts carrying digital
data such that, during the time-division two-way communication,
reception of a said burst from said other device is completed
before transmission of the next burst by said telecommunication
device is begun;
said telecommunication device comprising:
means for producing bursts for transmission by the
telecommunication device at least some of which are in a particular
format of digital data such that a burst in said particular format
comprises a 24-bit synchronisation pattern selected from the group,
defined in hexadecimal format: BE4E50; 41B1AF; EB1B05; 14E4FA;
0A727D; F58D82; A0D8D7; and 5F2728.
41. A telecommunication device for establishing time-division
two-way communication with another device over a radio channel by
exchanging radio signals in alternating bursts carrying digital
data such that, during the time-division two-way communication,
reception of a said burst from said other device is completed
before transmission of the next burst by said telecommunication
device is begun;
said telecommunication device comprising:
means for asynchronously detecting in a received burst a
synchronisation pattern selected from the group, defined in
hexadecimal format: BE4E50; 41B1AF; EB1B05; 14E4FA; 0A727D; F58D82;
A0D8D7; and 5F2728, to determine timing information about the
burst.
42. A system according to claim 13 wherein said first device
comprises a said means for producing bursts of said particular
format such that a particular burst of said particular format from
the first device comprises a first one of said plurality of
synchronisation patterns, and said second device comprises a said
means for producing bursts of said particular format such that a
particular burst of said particular format from the second device
comprises a second one of said plurality of synchronisation
patterns.
43. A system according to claim 13 wherein said burst producing
means comprises means to variably select different said preset
synchronisation patterns from among said plurality of
synchronisation patterns at different times.
44. A method according to claim 26 wherein said preset
synchronisation pattern comprises a first synchronisation pattern
or one of a first group of synchronisation patterns when said step
of producing bursts of said particular format is carried out in
said first device and said preset synchronisation pattern comprises
a second synchronisation pattern or one of a second group of
synchronisation patterns when said step of producing burst of said
particular format is carried out in said second device.
45. A method according to claim 26 wherein said preset
synchronisation pattern is a different one of said plurality of
synchronisation patterns on different occasions when said step of
producing particular bursts is carried out.
46. A system according to claim 8 wherein said bursts of said
particular format are not said alternating bursts.
47. A device according to claim 34 wherein said bursts of said
particular format are not said alternating bursts.
Description
FIELD OF THE INVENTION
The present invention relates to telecommunications systems, and
has particular application to cordless telephones. Aspects of the
invention are useful in so called "CT2" cordless telephone systems,
and systems in accordance with the British Department of Trade and
Industry specification MPT 1375. The May 1989 version of
specification MPT 1375 is incorporated herein by reference.
BACKGROUND
In a cordless telephone system, it is necessary to provide a way of
carrying the signal for the contents of communication, normally
speech, in both directions between parts of the system which are
not connected by a cord or wire. Additionally, it will normally be
necessary to pass other signals between the parts, which are used
to control the operation of the parts or carry other control
messages separate from the content of the communication. In some
known radio telephone systems, the requirement for two way
communication is achieved by providing two radio channels between
the parts, each channel being used for communication in one
respective direction. In an embodiment of the present invention,
multiplexed signal structures are provided enabling a plurality of
logical channels to be carried with communication in both
directions, over a single signal communications channel. In the
embodiment, the use of different multiplex structures at different
times permits differences in the logical channel structure of the
cordless communication at different stages of the creation and use
of the cordless communications link.
In a conventional radio telephone system, an arrangement must be
provided enabling a link to be set up so that parts can communicate
with each other. When one of the parts is operating in a manner
synchronised to some routine, it may be difficult for another part
to establish a link with the first part if the second part is not
itself synchronised to the same routine. In an embodiment of the
present invention, an arrangement is provided allowing for
asynchronous initiation of a link between two parts, even when one
of the parts is operating in a synchronous manner.
In a radio telecommunication system, the ability of a radio link to
carry useful signals will tend to vary in accordance with external
factors, such as interference and transmission past obstructions.
Accordingly, it is advantageous to encode transmitted signals for
error detection and correction and/or monitor the link quality to
enable remedial steps such as breaking and re-establishing the
link, possibly on a different radio channel, if the link quality
becomes unacceptably low. In an embodiment of the present
invention, an arrangement is provided in which two logical channels
are multiplexed together, with signals of one logical channel being
encoded to enable error detection, and detected errors in this
logical channel being monitored and used as a measure of the extent
to which the other channel is exposed to errors.
In a system for radio telecommunications, there will typically be a
large number of communication devices capable of communicating in
the system, some of which may be more sophisticated and have
greater communication abilities than others. In order for two
devices to communicate with each other, they must communicate in a
manner which is within the capabilities of both devices. Thus, when
a relatively sophisticated device communicates with an
unsophisticated device, they must communicate in a manner within
the capabilities of the unsophisticated device. However, it is
inefficient to force the sophisticated device also to communicate
in this particular manner when it is communicating with another
sophisticated device capable of communicating in a different
manner. In an embodiment of the present invention, devices conduct
an operation (sometimes referred to as a "negotiation" operation)
during the creation of a cordless telecommunications link, so as to
adopt a way of communicating which is within the capabilities of
both devices.
When two devices are communicating over a cordless
telecommunications link, it may be necessary for the operations of
the devices to be synchronised with each other and this may be done
by one device recognising a particular part of a signal transmitted
by the other device, the signal part having a predetermined timing.
In this case, incorrect synchronisation can arise if the receiving
part incorrectly identifies a different part of the transmitted
signal as the part to be recognised for synchronisation. In an
embodiment of the present invention, a signal is transmitted having
a data structure such that a portion used for synchronisation has a
low correlation with other portions of the signal not containing
the synchronisation part. Additionally, in the embodiment the
synchronisation part has a low correlation with time-shifted
versions of itself. Preferably, signal parts used for
synchronisation are transmitted in both directions between the
devices, and a signal part used for synchronisation and transmitted
in one direction is arranged to have a low correlation with a
signal part used for synchronisation and transmitted in the other
direction.
When a plurality of devices capable of communicating over a
cordless communications link are present in the same area, and
several are scanning communication channels to detect another
device seeking to set up a communications link, there is a
possibility that two devices may detect the same request for a
communications link on a channel and both respond to the request
simultaneously. The resulting interference on the channel may
result in neither device establishing the communications link. If
the subsequent behaviour of the two devices in scanning the
channels for requests for a communications link is identical, such
a simultaneous response and interference is likely to occur with
every subsequent detection of a communication request. In an
embodiment of the present invention, some devices are arranged so
as to have a behaviour following such a simultaneous response and
interference which is different from each other, so as to reduce
the likelihood of subsequent repetitions of the simultaneous
response and interference.
Devices communicating with each other over a cordless
telecommunications link may exchange "handshake" signals to confirm
that communication between them over the link is still taking place
successfully. If one of the devices fails to receive a handshake
signal within a certain period, it may conclude that the link has
been broken. However, the device which has ceased to receive
handshake signals will typically still be transmitting them until
the end of the period at which it concludes that the link has been
broken. If these handshake signals are successfully received by the
other device, then the other device will not become aware of the
failure of the link until a further period after the first device
ceases to transmit handshake signals. Thus, when a transmission
link fails in one direction only, the reaction of the devices may
be delayed and will typically not be synchronised with each other.
In an embodiment of the present invention, if a device fails to
receive a handshake signal within a first period of its most recent
receipt of a handshake signal, it concludes that the link has been
lost. In the meantime, it continues to transmit handshake signals,
but if it has not received a handshake signal within a second,
shorter, period since the most recent handshake signal, it
transmits a signal indicating that it is failing to receive
handshake signals. Thus, if a link breaks down in one direction
only, the device which is continuing to receive transmissions is
rapidly notified that the other device has ceased to receive
transmissions, and the link re-establishment actions of the devices
can be better co-ordinated.
Where two devices communicate with each other in a synchronised
manner, it is possible for the transmission of information to
become corrupted by a loss of synchronisation, even though the
transmission quality of the communication link may be unimpaired.
In an embodiment of the present invention, some of the transmitted
information is coded to enable error detection, and the detection
of errors in this data can be used as an indication that
synchronisation between the devices has been lost.
When two devices are communicating with each other over a
communications link, in a synchronised manner, one of them may be
designated a synchronisation master, and the other a
synchronisation slave, such that the slave is required to
synchronise itself to the operations of the master. If the link
fails, or for any other reason the devices are required to break
and re-establish the link, re-establishment may be difficult if the
slave device ceases to be synchronised to the master and therefore
fails to detect link re-establishment signals from the master. In
an embodiment of the present invention, when link re-establishment
is required the initial signalling to carry out the
re-establishment is always transmitted by the slave device.
If one of the devices in a link is portable or mobile, the link may
be broken by the movement of that device. It may then be impossible
to re-establish the link between the same two devices. If one of
the devices is also an endpoint of a communications path, e.g. a
handset, and the other is only a relay station, e.g. a base station
linked to a communications network, it is preferable to
re-establish the link using the same endpoint device, but possibly
a different relay device, for the convenience of the user. However,
it may be difficult for the communications network to monitor which
relay device an endpoint device is near at any given time. In an
embodiment of the present invention the endpoint device transmits
the initial signals in link re-establishment, and the link can be
re-established using any relay device which receives the
transmissions. The endpoint device is typically the mobile device,
e.g. a portable telephone handset.
When devices are communicating over a communications link using an
alternating transmission burst arrangement, there may be a failure
of communication if the timings of the transmissions of the two
devices are not properly co-ordinated and their transmissions
partially overlap instead of alternating correctly. In an
embodiment of the present invention one device derives the timing
for the transmission of a burst from the time at which it receives
a burst from the other device.
When devices communicate over a telecommunications link, it may be
necessary to transmit signals belonging to a logical channel for
the purposes of link maintenance, even though there is no
information to be transmitted in that channel at that time. If a
random signal is sent in that logical channel under the
circumstances, it may by chance resemble some meaningful signal
transmitted over the communications link, resulting in incorrect
operation of the device receiving the signal. In an embodiment of
the present invention, a specified signal structure is provided for
a logical channel which structure conveys no useful information in
that channel, but which is chosen not to resemble a signal the
reception of which could cause incorrect operation of the receiving
device.
SUMMARY OF THE INVENTION
In accordance with an aspect of the present invention there is
provided a telecommunication system in which first and second
devices are capable of synchronous time-division two-way
communication with each other over a radio channel by exchanging
radio signals in alternating bursts carrying digital data, such
that during the time-division two-way communication transmission of
a said burst from one of the first and second devices is completed
before transmission of the next burst by the other of the first and
second devices is begun,
characterised in that:
first and second formats of digital data in the said bursts are
used in said time-division two-way communication, both of which
formats include information for a first logical communication
channel which carries signalling data such as device identification
codes and instructions from one device to the other;
the first said format also includes information for a second
logical communication channel which carries data, such as digitally
encoded speech, which is to be communicated between the devices;
and
the second said format also includes a synchronisation pattern not
included in the first format, the synchronisation pattern enabling
a said device to determine the timing of the bursts received by it
from the other said device.
In accordance with another aspect of the present invention there is
provided a method of telecommunication in which first and second
devices perform synchronous time-division two-way communication
with each other over a radio channel by exchanging radio signals in
alternating bursts carrying digital data, such that during the
time-division two-way communication transmission of a said burst
from one of the first and second devices is completed before
transmission of the next burst by the other of the first and second
devices is begun,
characterised in that:
first and second formats of digital data in the said bursts are
used in said time-division two-way communication, both of which
formats include information for a first logical communication
channel which carries signalling data such as device identification
codes and instructions from one device to the other;
the first said format also includes information for a second
logical communication channel which carries data, such as digitally
encoded speech, which is to be communicated between the devices;
and
the second said format also includes a synchronisation pattern not
included in the first format, the synchronisation pattern enabling
a said device to determine the timing of the bursts received by it
from the other said device.
Preferably, in the said first format, information for the first
logical communication channel is transmitted both before and after
information for the second logical communication channel.
Preferably, in the said first format, the same number of bits of
information for the first channel are transmitted before the
information for the second channel as are transmitted after the
information for the second channel.
Preferably, in the said second format, information for the first
logical communication channel is transmitted both before and after
the said synchronisation pattern.
Preferably, in the said second format, the same number of bits of
information for the first channel are transmitted before the said
synchronisation pattern as are transmitted after the said
synchronisation pattern.
Preferably, more bits of information for the first channel are
transmitted in a burst of the second format than in a burst of the
first format.
Preferably, more bits of information for the second channel are
transmitted in a burst of the first format than the number of bits
of information for the first channel in a burst of either the first
or the second format.
In accordance with another aspect of the present invention there is
provided a telecommunication system in which first and second
devices are capable of synchronous time-division two-way
communication with each other over a radio channel by exchanging
radio signals in alternating bursts carrying digital data, such
that during the time-division two-way communication transmission of
a said burst from one of the first and second devices is completed
before transmission of the next burst by the other of the first and
second devices is begun,
characterised in that:
the said second device is also capable of transmitting
asynchronously a burst comprising one or more portions of digital
data for a logical communication channel, which carries signalling
data such as device identification codes, followed by one or more
further portions of digital data, after which the second device
ceases transmission for a period to enable it to receive a reply
from the first device, each respective said portion or further
portion of digital data comprising a plurality of occurrences of a
respective digital data sequence, and the digital data sequence for
each said further portion of digital data providing a
synchronisation pattern to enable the said first device to
determine the timing of the burst when it receives it.
In accordance with another aspect of the present invention there is
provided a method of telecommunication in which first and second
devices perform time-division two-way communication with each other
over a radio channel by exchanging radio signals in alternating
bursts carrying digital data, such that during the time-division
two-way communication transmission of a said burst from one of the
first and second devices is completed before transmission of the
next burst by the other of the first and second devices is
begun,
characterised in that:
at a time when the said first and second devices are not performing
the time division two-way communication, the second device
transmits asynchronously a burst comprising one or more portions of
digital data for a logical communication channel, which carries
signalling data such as device identification codes, followed by
one or more further portions of digital data, after which the
second device ceases transmission for a period to enable it to
receive a reply from the first device, each respective said portion
or further portion of digital data comprising a plurality of
occurrences of a respective digital data sequence, and the digital
data sequence for each said further portion of digital data
providing a synchronisation pattern to enable the said first device
to determine the timing of the burst when it receives it.
Preferably, the said bursts exchanged in the said time division
two-way communication are exchanged in a succession of burst
periods all of the same length, a burst being transmitted from the
first device to the second device and a burst being transmitted
from the second device to the first device at different times in a
burst period, and each said portion or further portion of digital
data in a said asynchronously transmitted burst lasting for at
least the length of a said burst period, and each said digital data
sequence lasting for no more than half the length of a part of a
said burst period in which part the first device does not transmit
a burst.
In accordance with another aspect of the present invention there is
provided a telecommunication system in which first and second
devices are capable of time-division two-way communication with
each other over a radio channel by exchanging radio signals in
alternating bursts carrying digital data, such that during the
time-division two-way communication transmission of a said burst
from one of the first and second devices is completed before
transmission of the next burst by the other of the first and second
devices is begun,
characterised in that:
in bursts of a first type information is transmitted for both a
first logical communication channel, which carries signalling data
such as device identification codes and instructions from one
device to the other, and a second logical communication channel,
which carries data to be communicated between the devices such as
digitally encoded speech,
and the first and second devices conduct an operation to select, in
accordance with their capabilities, a format for bursts of the
first type from a predefined set of formats comprising first and
second formats which differ in the amount of information for the
first logical communication channel carried in each burst.
In accordance with another aspect of the present invention there is
provided a method of telecommunication in which first and second
devices perform time-division two-way communication with each other
over a radio channel by exchanging radio signals in alternating
bursts carrying digital data, such that during the time-division
two-way communication transmission of a said burst from one of the
first and second devices is completed before transmission of the
next burst by the other of the first and second devices is
begun,
characterised in that:
in bursts of a first type information is transmitted for both a
first logical communication channel, which carries signalling data
such as device identification codes and instructions from one
device to the other, and a second logical communication channel,
which carries data to be communicated between the devices such as
digitally encoded speech,
and the first and second devices conduct an operation to select, in
accordance with their capabilities, a format for bursts of the
first type from a predefined set of formats comprising first and
second formats which differ in the amount of information for the
first logical communication channel carried in each burst.
Preferably, the said alternating bursts are transmitted in burst
periods, one burst being transmitted in each direction at different
times in a burst period, and the said first and second formats
require different lengths of time for transmission of a burst but
the same length of time for a burst period.
Preferably, the said first and second formats carry the same amount
of the second logical communication channel per burst as each
other.
In accordance with another aspect of the present invention there is
provided a telecommunication system in which first and second
devices are capable of time-division two-way communication with
each other over a radio channel by exchanging radio signals in
alternating bursts carrying digital data, such that during the
time-division two-way communication transmission of a said burst
from one of the first and second devices is completed before
transmission of the next burst by the other of the first and second
devices is begun,
characterised in that:
at least some bursts comprise information for a first logical
channel and information for second logical channel at different
times in the burst, the data of the first logical channel being
structured so as to enable the detection of transmission errors,
and each said device using detected errors in the first logical
channel in bursts received by it as an indication of the quality of
transmission of the second logical channel.
In accordance with another aspect of the present invention there is
provided a method of telecommunication in which first and second
devices perform time-division two-way communication with each other
over a radio channel by exchanging radio signals in alternating
bursts carrying digital data, such that during the time-division
two-way communication transmission of a said burst from one of the
first and second devices is completed before transmission of the
next burst by the other of the first and second devices is
begun,
characterised in that:
at least some bursts comprise information for a first logical
channel and information for second logical channel at different
times in the burst, the data of the first logical channel being
structured so as to enable the detection of transmission errors,
and each said device using detected errors in the first logical
channel in bursts received by it as an indication of the quality of
transmission of the second logical channel.
Preferably, information for the first logical channel is provided
in a said burst both before and after information for the second
logical channel.
Preferably, if the quality of transmission of the second logical
channel, as determined by a said device using detected errors in
the first logical channel, fails to meet a predetermined criterion,
the device enters a mode to re-establish the said time-division
two-way communication.
Preferably, the device which determines the failure of the quality
of transmission of the second logical channel to meet the
predetermined criterion sends a message to the other device before
it enters the said mode, to inform the other device that it is
about to enter the said mode.
Preferably, the said devices do not structure the data of the
second logical channel to enable the detection of transmission
errors.
In accordance with another aspect of the present invention there is
provided a telecommunication system in which first and second
devices are capable of time-division two-way communication with
each other over a radio channel by exchanging radio signals in
alternating bursts carrying digital data, such that during the
time-division two-way communication transmission of a said burst
from one of the first and second devices is completed before
transmission of the next burst by the other of the first and second
devices is begun,
characterised in that:
each of the first and second devices sends repeatedly one of a
pre-set group of signal codes while they are exchanging the said
radio signals, at a rate such that the period between successive
transmissions by the same device of one of the pre-set group of
codes does not exceed a first predetermined length of time, a first
code of said group of codes normally being sent if the sending
device has received any of said group of codes within the first
predetermined length of time before the time it sends the code, and
a second code of said group of codes normally being sent otherwise,
and each of the first and second devices entering a mode for
re-establishing the time-division two-way communication if it has
not received the said first code for a second predetermined length
of time greater than the first.
In accordance with another aspect of the present invention there is
provided a method of telecommunication in which first and second
devices perform time-division two-way communication with each other
over a radio channel by exchanging radio signals in alternating
bursts carrying digital data, such that during the time-division
two-way communication transmission of a said burst from one of the
first and second devices is completed before transmission of the
next burst by the other of the first and second devices is
begun,
characterised in that:
each of the first and second devices sends repeatedly one of a
pre-set group of signal codes while they are exchanging the said
radio signals, at a rate such that the period between successive
transmissions by the same device of one of the pre-set group of
codes does not exceed a first predetermined length of time, a first
code of said group of codes normally being sent if the sending
device has received any of said group of codes within the first
predetermined length of time before the time it sends the code, and
a second code of said group of codes normally being sent otherwise,
and each of the first and second devices entering a mode for
re-establishing the time-division two-way communication if it has
not received the said first code for a second predetermined length
of time greater than the first.
In accordance with another aspect of the present invention there is
provided a telecommunication system in which a remote unit is
capable of time-division two-way communication with a base station
over a radio channel by exchanging radio signals in alternating
bursts carrying digital data, such that during the time-division
two-way communication transmission of a said burst from one of the
remote unit and the base station is completed before transmission
of the next burst by the other of the remote unit and the base
station is begun, to enable the remote unit to communicate via the
base station with a further device,
characterised in that:
if either the remote unit or the base station with which it is in
said time-division two-way communication decides that
re-establishment of the time-division two-way communication is
required, it takes steps to cause the remote unit to transmit radio
signals to initiate the said re-establishment.
In accordance with another aspect of the present invention there is
provided a method of telecommunication in which a remote unit and a
base station perform time-division two-way communication with each
other over a radio channel by exchanging radio signals in
alternating bursts carrying digital data, such that during the
time-division two-way communication transmission of a said burst
from one of the remote unit and the base station is completed
before transmission of the next burst by the other of the remote
unit and the base station is begun, to enable the remote unit to
communicate via the base station with a further device,
characterised in that:
if either the remote unit or the base station with which it is in
said time-division two-way communication decides that
re-establishment of the time-division two-way communication is
required, it takes steps to cause the remote unit to transmit radio
signals to initiate the said re-establishment.
Preferably, the handset is capable of the said time-division
two-way communication with a plurality of base stations, so that
the said re-establishment takes place between the said remote unit
and a said base station which is not necessarily the same base
station as the remote unit was previously in said communication
with.
Preferably, the said radio signals transmitted by the remote unit
to initiate the said re-establishment comprise the radio signals
transmitted by the remote unit to initiate the establishment of
said communication when the remote unit has not been in said
communication immediately beforehand, modified so as to convey an
identification of the time-division two-way communication to be
re-established.
In accordance with another aspect of the present invention there is
provided a telecommunication system in which first and second
devices are capable of time-division two-way communication with
each other over a radio channel by exchanging radio signals in
alternating bursts carrying digital data, such that during the
time-division two-way communication transmission of a said burst
from one of the first and second devices is completed before
transmission of the next burst by the other of the first and second
devices is begun,
characterised in that:
at least some of the information communicated between the devices
over a first logical channel is structured in words which include
an error detection code, and a predetermined word is defined for
communication between the devices which includes a said error
detection code but carries substantially no messages from the
transmitting device to the receiving device.
In accordance with another aspect of the present invention there is
provided a method of telecommunication in which first and second
devices perform time-division two-way communication with each other
over a radio channel by exchanging radio signals in alternating
bursts carrying digital data, such that during the time-division
two-way communication transmission of a said burst from one of the
first and second devices is completed before transmission of the
next burst by the other of the first and second devices is
begun,
characterised in that:
at least some of the information communicated between the devices
over a first logical channel is structured in words which include
an error detection code, and a predetermined word is defined for
communication between the devices which includes a said error
detection code but carries substantially no messages from the
transmitting device to the receiving device.
Preferably, the transmission of a first type of said words is
preceded by the transmission over the first logical channel of a
set pattern indicating the timing of the following word, and no
part of the predetermined word includes data in the same sequence
as the said set pattern.
In accordance with another aspect of the present invention there is
provided a telecommunication system in which first and second
devices are capable of time-division two-way communication with
each other over a radio channel by exchanging radio signals in
alternating bursts carrying digital data, such that during the
time-division two-way communication transmission of a said burst
from one of the first and second devices is completed before
transmission of the next burst by the other of the first and second
devices is begun,
characterised in that:
at least some of the information communicated between the devices
over a first logical channel is structured in words, and
transmission of a first type of said words is preceded by
transmission over the first logical channel of a set pattern
indicating the timing of the following word,
and in between repetitions of words of the first type carrying the
same messages or parts of messages and having the same data
sequence there is transmitted over the first logical channel either
a different word of the first type or a predefined sequence which
does not include the said set pattern, whereby if the data sequence
of a repeated said word of the first type includes the said set
pattern it is repeated sufficiently infrequently to allow the
receiving device to identify the said set pattern correctly in the
transmission of the set pattern before the next transmission of the
repeated word even if the receiving device incorrectly identifies
the set pattern in the data sequence of the previous transmission
of the repeated word as an occurrence of the set pattern preceding
a word of the first type.
In accordance with another aspect of the present invention there is
provided a method of telecommunication in which first and second
devices perform time-division two-way communication with each other
over a radio channel by exchanging radio signals in alternating
bursts carrying digital data, such that during the time-division
two-way communication transmission of a said burst from one of the
first and second devices is completed before transmission of the
next burst by the other of the first and second devices is
begun,
characterised in that:
at least some of the information communicated between the devices
over a first logical channel is structured in words, and
transmission of a first type of said words is preceded by
transmission over the first logical channel of a set pattern
indicating the timing of the following word,
and in between repetitions of words of the first type carrying the
same messages or parts of messages and having the same data
sequence there is transmitted over the first logical channel either
a different word of the first type or a predefined sequence which
does not include the said set pattern, whereby if the data sequence
of a repeated said word of the first type includes the said set
pattern it is repeated sufficiently infrequently to allow the
receiving device to identify the said set pattern correctly in the
transmission of the set pattern before the next transmission of the
repeated word even if the receiving device incorrectly identifies
the set pattern in the data sequence of the previous transmission
of the repeated word as an occurrence of the set pattern preceding
a word of the first type.
In accordance with another aspect of the present invention there is
provided a telecommunication system in which a device of a first
type is capable of time-division two-way communication with any one
of a plurality of devices of a second type over a radio channel by
exchanging radio signals in alternating bursts carrying digital
data, such that during the time-division two-way communication
transmission of a said burst from one of the devices is completed
before transmission of the next burst by the other of the devices
is begun,
characterised in that:
at least some of the bursts contain a synchronisation pattern which
may be detected asynchronously by the receiving device to enable it
to discover the timing of the received burst, the device of the
first type transmitting a first synchronisation pattern or one of a
group of first synchronisation patterns and the devices of the
second type transmitting a second synchronisation pattern or one of
a group of second synchronisation patterns, the first
synchronisation pattern or patterns being different from the second
synchronisation pattern or patterns and the devices of the second
type not responding to reception of the or a second synchronisation
pattern, whereby devices of the second type do not respond to
reception of transmissions by other devices of the second type.
In accordance with another aspect of the present invention there is
provided a method of telecommunication in which a device of a first
type and any one of a plurality of devices of a second type perform
time-division two-way communication with each other over a radio
channel by exchanging radio signals in alternating bursts carrying
digital data, such that during the time-division two-way
communication transmission of a said burst from one of the devices
is completed before transmission of the next burst by the other of
the devices is begun,
characterised in that:
at least some of the bursts contain a synchronisation pattern which
may be detected asynchronously by the receiving device to enable it
to discover the timing of the received burst, the device of the
first type transmitting a first synchronisation pattern or one of a
group of first synchronisation patterns and the devices of the
second type transmitting a second synchronisation pattern or one of
a group of second synchronisation patterns, the first
synchronisation pattern or patterns being different from the second
synchronisation pattern or patterns and the devices of the second
type not responding to reception of the or a second synchronisation
pattern, whereby devices of the second type do not respond to
reception of transmissions by other devices of the second type.
Preferably, the said time-division two-way communication may be
performed by any one of a plurality of devices of the first type,
and the devices of the first type do not respond to reception of
the or a first synchronisation pattern, whereby devices of the
first type do not respond to transmissions by other devices of the
first type.
Preferably, each said device transmits a predetermined
synchronisation pattern while attempting to initiate communication
by said radio signals with a device of the other type, and
subsequently transmits a different predetermined synchronisation
pattern after said communication has been initiated.
In accordance with another aspect of the present invention there is
provided a telecommunication system in which first and second
devices are capable of time-division two-way communication with
each other over a radio channel by exchanging radio signals in
alternating bursts carrying digital data, such that during the
time-division two-way communication transmission of a said burst
from one of the first and second devices is completed before
transmission of the next burst by the other of the first and second
devices is begun,
characterised in that:
at least some of the bursts transmitted by a said device contain a
synchronisation pattern of bits of data, which may be detected
asynchronously by the receiving device to enable it to discover the
timing of the transmitted burst, and also contain bits of variable
data, and the receiving device deems the synchronisation pattern to
be present in the received data when a comparison operation between
the received data and a stored copy of the synchronisation pattern
results in no more than K bits of the received data failing the
comparison, where K is zero or a positive integer, and the
arrangement of bits in each said burst being such that in any
consecutive string of L bits of data in the burst, there are less
than L-K bits of variable data.
In accordance with another aspect of the present invention there is
provided a method of telecommunication in which first and second
devices perform time-division two-way communication with each other
over a radio channel by exchanging radio signals in alternating
bursts carrying digital data, such that during the time-division
two-way communication transmission of a said burst from one of the
first and second devices is completed before transmission of the
next burst by the other of the first and second devices is
begun,
characterised in that:
at least some of the bursts transmitted by a said device contain a
synchronisation pattern of bits of data, which may be detected
asynchronously by the receiving device to enable it to discover the
timing of the transmitted burst, and also contain bits of variable
data, and the receiving device deems the synchronisation pattern to
be present in the received data when a comparison operation between
the received data and a stored copy of the synchronisation pattern
results in no more than K bits of the received data failing the
comparison, where K is zero or a positive integer, and the
arrangement of bits in each said burst being such that in any
consecutive string of L bits of data in the burst, there are less
than L-K bits of variable data.
Preferably, the arrangement of bits in each said burst is such that
in any consecutive string of L bits of data in the burst, there are
no more than L-K-6 bits of variable data.
In accordance with another aspect of the present invention there is
provided a telecommunication system in which first and second
devices are capable of time-division two-way communication with
each other over a radio channel by exchanging radio signals in
alternating bursts carrying digital data, such that during the
time-division two-way communication transmission of a said burst
from one of the first and second devices is completed before
transmission of the next burst by the other of the first and second
devices is begun,
characterised in that:
at least one format of digital data in a burst comprises a first
portion having a repeating pattern of fixed value and variable bits
and a second portion comprising an L-bit synchronisation pattern
which may be detected asynchronously by the receiving device to
enable it to discover the timing of the burst, and the receiving
device deems the synchronisation pattern to be present in the
received data when a comparison operation between the received data
and a stored copy of the synchronisation pattern results in no more
than K bits of the received data failing the comparison, where K is
zero or a positive integer,
the L-bit synchronisation pattern and the repeating pattern of
fixed value and variable bits being selected such that a string of
L successive bits of the repeating pattern, starting at any
position in a repeat of the pattern, matches less than L-K bits of
the synchronisation pattern even if it is assumed that every
variable bit in the string provides a match.
In accordance with another aspect of the present invention there is
provided a method of telecommunication in which first and second
devices perform time-division two-way communication with each other
over a radio channel by exchanging radio signals in alternating
bursts carrying digital data, such that during the time-division
two-way communication transmission of a said burst from one of the
first and second devices is completed before transmission of the
next burst by the other of the first and second devices is
begun,
characterised in that:
at least one format of digital data in a burst comprises a first
portion having a repeating pattern of fixed value and variable bits
and a second portion comprising an L-bit synchronisation pattern
which may be detected asynchronously by the receiving device to
enable it to discover the timing of the burst, and the receiving
device deems the synchronisation pattern to be present in the
received data when a comparison operation between the received data
and a stored copy of the synchronisation pattern results in no more
than K bits of the received data failing the comparison, where K is
zero or a positive integer,
the L-bit synchronisation pattern and the repeating pattern of
fixed value and variable bits being selected such that a string of
L successive bits of the repeating pattern, starting at any
position in a repeat of the pattern, matches less than L-K bits of
the synchronisation pattern even if it is assumed that every
variable bit in the string provides a match.
Preferably, any said string of L successive bits of the said
repeating pattern matches no more than L-K-2 bits of the
synchronisation pattern even if it is assumed that every variable
bit of the string provides a match.
In accordance with another aspect of the present invention there is
provided a telecommunication system in which first and second
devices are capable of time-division two-way communication with
each other over a radio channel by exchanging radio signals in
alternating bursts carrying digital data, such that during the
time-division two-way communication transmission of a said burst
from one of the first and second devices is completed before
transmission of the next burst by the other of the first and second
devices is begun,
characterised in that:
at least one format of digital data in a burst comprises an L-bit
synchronisation pattern which may be detected asynchronously by a
receiving device to enable it to discover the timing of the burst,
and the receiving device deems the synchronisation pattern to be
present in the received data when a comparison operation between
the received data and a stored copy of the synchronisation pattern
results in no more than K bits of the received data failing the
comparison, where K is zero or a positive integer,
the synchronisation pattern being adjacent a portion of the burst
made up of fixed value bits, and the number of matches between the
synchronisation pattern and any string of L successive bits of the
burst composed only of least a part of the said portion of fixed
value bits and an adjacent part of the synchronisation pattern
being less than L-K.
In accordance with another aspect of the present invention there is
provided a method of telecommunication in which first and second
devices perform time-division two-way communication with each other
over a radio channel by exchanging radio signals in alternating
bursts carrying digital data, such that during the time-division
two-way communication transmission of a said burst from one of the
first and second devices is completed before transmission of the
next burst by the other of the first and second devices is
begun,
characterised in that:
at least one format of digital data in a burst comprises an L-bit
synchronisation pattern which may be detected asynchronously by a
receiving device to enable it to discover the timing of the burst,
and the receiving device deems the synchronisation pattern to be
present in the received data when a comparison operation between
the received data and a stored copy of the synchronisation pattern
results in no more than K bits of the received data failing the
comparison, where K is zero or a positive integer,
the synchronisation pattern being adjacent a portion of the burst
made up of fixed value bits, and the number of matches between the
synchronisation pattern and any string of L successive bits of the
burst composed only of least a part of the said portion of fixed
value bits and an adjacent part of the synchronisation pattern
being less than L-K.
Preferably, different said L-bit synchronisation patterns are used
under different circumstances, and the number of matches between
any said synchronisation pattern and any string of L successive
bits of the burst composed only of any other said synchronisation
pattern or at least a part of the said portion of fixed value bits
and an adjacent part of any other said synchronisation pattern is
less than L-K.
Preferably, for at least some of the said synchronisation patterns,
the said number of matches does not exceed L-K-8.
Preferably, for all of the said synchronisation patterns, the said
number of matches does not exceed L-K-7.
Preferably, K is not zero and more preferably, K is two.
In accordance with another aspect of the present invention there is
provided a telecommunication system in which first and second
devices are capable of time-division two-way communication with
each other over a radio channel by exchanging radio signals in
alternating bursts carrying digital data, such that during the
time-division two-way communication transmission of a said burst
from one of the first and second devices is completed before
transmission of the next burst by the other of the first and second
devices is begun,
characterised in that:
at least one format of digital data in a burst comprises an L-bit
synchronisation pattern which may be detected asynchronously by a
receiving device to enable it to discover the timing of the burst,
the synchronisation pattern having a peak self-correlation side
lobe value of not more than +2, for any amount of offset, where the
self-correlation side lobe value at an amount of offset is defined
as the number of matches between bits of the pattern and itself
offset by the amount, minus the number of mismatches between the
bits of the pattern and itself at the same amount of offset.
In accordance with another aspect of the present invention there is
provided a method of telecommunication in which first and second
devices perform time-division two-way communication with each other
over a radio channel by exchanging radio signals in alternating
bursts carrying digital data, such that during the time-division
two-way communication transmission of a said burst from one of the
first and second devices is completed before transmission of the
next burst by the other of the first and second devices is
begun,
characterised in that:
at least one format of digital data in a burst comprises an L-bit
synchronisation pattern which may be detected asynchronously by a
receiving device to enable it to discover the timing of the burst,
the synchronisation pattern having a peak self-correlation side
lobe value of not more than +2, for any amount of offset, where the
self-correlation side lobe value at an amount of offset is defined
as the number of matches between bits of the pattern and itself
offset by the amount, minus the number of mismatches between the
bits of the pattern and itself at the same amount of offset.
In accordance with another aspect of the present invention there is
provided a telecommunication system in which first and second
devices are capable of time-division two-way communication with
each other over a radio channel by exchanging radio signals in
alternating bursts carrying digital data, such that during the
time-division two-way communication transmission of a said burst
from one of the first and second devices is completed before
transmission of the next burst by the other of the first and second
devices is begun,
characterised in that:
at least one format of digital data in a burst comprises a 24-bit
synchronisation pattern which may be detected asynchronously by a
receiving device to enable it to discover the timing of the burst,
the synchronisation pattern, when given in hexadecimal format,
being one of: BE4E50; 41B1AF; EB1B05; 14E4FA; 0A727D; F58D82;
A0D8D7; and 5F2728.
In accordance with another aspect of the present invention there is
provided a method of telecommunication in which first and second
devices perform time-division two-way communication with each other
over a radio channel by exchanging radio signals in alternating
bursts carrying digital data, such that during the time-division
two-way communication transmission of a said burst from one of the
first and second devices is completed before transmission of the
next burst by the other of the first and second devices is
begun,
characterised in that:
at least one format of digital data in a burst comprises a 24-bit
synchronisation pattern which may be detected asynchronously by a
receiving device to enable it to discover the timing of the burst,
the synchronisation pattern, when given in hexadecimal format,
being one of: BE4E50; 41B1AF; EB1B05; 14E4FA; 0A727D; F58D82;
A0D8D7; and 5F2728.
The present invention also includes a communication device usable
in any of the above systems, and in particular a communication
device usable as the first device and a communication device usable
as the second device.
BRIEF DESCRIPTION OF THE DRAWINGS
Embodiments of the present invention, given by way of example, will
now be described with reference to the accompanying drawings in
which:
FIG. 1 is a schematic view of a telecommunications system connected
to a base station of a system embodying the present invention;
FIG. 2 is a schematic illustration of the pattern of burst
transmissions in an embodiment of the present invention;
FIG. 3 is a schematic representation of the frequency and amplitude
variation of the radio frequency signal in a burst in an embodiment
of the present invention;
FIGS. 4a and 4b illustrate schematically first and second versions
of a first type of data structure in a signal burst used in an
embodiment of the present invention;
FIG. 5 illustrates schematically a second type of data structure in
a signal burst used in an embodiment of the present invention;
FIG. 6 illustrates schematically the relative timing of a third
type of data structure, transmitted by the handset in an embodiment
of the present invention, relative to the transmission cycle of a
base station in the embodiment of the present invention;
FIG. 7 illustrates in more detail a portion of FIG. 6;
FIG. 8 illustrates schematically the arrangement of data in the
data structure of FIG. 6;
FIG. 9 illustrates in more detail part of FIG. 8;
FIG. 10 illustrates a first version of a handset for use in an
embodiment of the present invention;
FIG. 11 illustrates a second version of a handset used in an
embodiment of the present invention;
FIG. 12 illustrates schematically the parts of a handset in an
embodiment of the present invention;
FIG. 13 illustrates schematically a version of a base station for
use in an embodiment of the present invention;
FIG. 14 illustrates schematically the parts of a base station for
use in an embodiment of the present invention;
FIG. 15 is a schematic block diagram of the control circuit of a
handset;
FIG. 16 is a schematic block diagram of the control circuit of the
base station;
FIG. 17 is a schematic block diagram of the programmable
multiplexer of FIGS. 15 and 16;
FIG. 18 is a schematic block diagram of the programmable
demultiplexer of FIGS. 15 and 16;
FIG. 19 is a schematic block diagram of the system controller of
FIGS. 15 and 16;
FIG. 20 is a schematic block diagram of the S channel controller of
FIGS. 15 and 16;
FIG. 21 is a flow diagram for the setting up of a link from a base
station to a handset;
FIG. 22 is a schematic representation of the sequence of signals
transmitted when a link is set up from a base station to a
handset;
FIG. 23 is a flow diagram for the setting up of a link from a
handset to a base station;
FIG. 24 is a schematic representation of the sequence of signals
transmitted when a link is set up from a handset to a base
station;
FIG. 25 is an overall view of the structure of data in the D
channel;
FIG. 26 shows schematically how D channel code words may be
assembled into packets and packets into messages;
FIG. 27 illustrates schematically the general format of a D channel
code word;
FIG. 28 illustrates schematically the format of a fixed format type
address code word of the D channel;
FIG. 29 illustrates schematically the format of a variable format
type address code word of the D channel;
FIG. 30 illustrates schematically the format of a data code word of
the D channel;
FIG. 31 illustrates schematically the structure of a fixed length
message in the D channel;
FIG. 32 illustrates schematically the structure of a variable
length message in the D channel;
FIG. 33 illustrates schematically the sequence of handshake signals
leading to link re-establishment when handshake is lost only from a
handset to a base station;
FIG. 34 illustrates schematically the sequence of handshake signals
leading to link re-establishment when handshake is lost only from a
base station to a handset;
FIG. 35 is a flow diagram of the link quality monitoring process
carried out in a handset or a base station;
FIG. 36 illustrates schematically the structure of a FILL-IN word
in the D channel;
FIG. 37 is a view similar to FIG. 1, illustrating alternative types
of base station for use in embodiments of the present
invention;
FIG. 38 illustrates in more detail a first alternative structure
for a base station embodying the present invention;
FIG. 39 illustrates in more detail a second alternative structure
for a base station embodying the present invention;
FIG. 40 illustrates a third alternative structure for a base
station embodying the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
In this specification, "kbit" and "kword" will be used as
abbreviations for "kilobits" and "kilowords", meaning "thousand
bits" and "thousand words" respectively, in accordance with normal
practice in the digital signalling art.
Overview
FIG. 1 shows schematically a number of telecommunications devices
connected to a telecommunications network 1. The telecommunications
network 1 is typically a PSTN (Public Switched Telephone Network),
although in the future an ISDN (Integrated Services Digital
Network) may be more common. The telecommunications network 1 is
connected to a variety of devices, and as examples of these FIG. 1
shows base stations 3 of a cordless telephone apparatus embodying
the present invention, a telephone and other telecommunications
apparatus 7 such as a facsimile machine or a modem for a computer.
Each of these communications devices is connected to the
telecommunications network 1 by a network link 9, which will
typically include some or all of a wired link, an optical fibre
link and a long distance radio link.
As shown in FIG. 1, the cordless telephone apparatus embodying the
present invention comprises a base station 3 and a remote unit 11.
The remote unit will typically be a telephone handset for speech
communication, and will be referred to hereinafter as a handset.
The handset 11 communicates with the base station 3 over a radio
link, diagrammatically represented at 13 in FIG. 1, and this
provides access for the user of the handset 11 to the
telecommunications network 1.
Although FIG. 1 shows only one handset 11 per base station 3,
alternative embodiments will be referred to with respect to other
figures, in which a plurality of handsets 11 are associated with
one base station 3, either so that the base station 3 can
communicate with any chosen one of the handsets 11, or in some
cases so that the base station 3 can communicate simultaneously
with different handsets 11 on different radio channels. The typical
uses of such cordless telephone systems include:
the provision of public telepoint services, in which carriers of
handsets 11 can go to the vicinity of a publicly available base
station 3, set up a radio link with the base station 3, and thereby
access the telecommunications network 1;
the provision of an office private branch exchange and/or intercom
system, having separately numbered extensions; and
the provision of a domestic or office extension and/or intercom
system in which extensions are not individually numbered.
For purely intercom use, there would, of course, be no need for the
base station 3 to be connected to a telecommunications network
1.
The construction and operation of the cordless telephone system
comprising the base station 3 and the handset 11 will now be
described. The system is in accordance with the British Department
of Trade and Industry specification MPT 1375, incorporated herein
by reference, to which reference may be had for further details and
regulatory restrictions. In MPT 1375, the base station 3 is
normally referred to as "the cordless fixed part" or "the CFP", and
the handset 11 is normally referred to as "the cordless portable
part" or "the CPP", and where appropriate similar terminology will
be used herein. The terms "fixed" and "portable" refer to the
nature of the typical connections made between the parts and the
telecommunications network 1, and do not necessarily mean that the
base station 3 cannot move from place to place.
Burst Mode Transmission and Timing
The base station 3 and the handset 11 communicate with each other
over a single radio channel, using time division duplex burst mode
transmission. The pattern of transmissions in an established radio
link is illustrated in FIG. 2. The top line in FIG. 2 represents
transmissions by the base station 3, and the lower line represents
transmissions by the handset 11.
A complete period in the burst mode transmission system lasts for
two milliseconds. Two bursts of data, one in each direction, are
transmitted in each burst period. Each two millisecond burst period
begins at the time t1 in FIG. 2, when the base station 3 begins to
transmit a burst. The burst transmission from the base station 3
ends at time t2, which is slightly less than 1 millisecond after
time t1. After a slight gap, the handset 11 begins a transmission
burst at time t3, and this transmission burst also lasts for
slightly less than 1 millisecond and ends at time t4. After another
slight gap, the next 2 millisecond transmission period begins.
The length of the transmission periods is controlled by the base
station 3, which ensures that successive times t1, at which it
begins transmitting a burst, are 2 milliseconds apart. The base
station 3 and the handset 11 both transmit digital data at 72
kilobits per second. Therefore each 2 ms burst period is equivalent
to 144 bit periods. Depending on the burst structure being used, as
will be described later, each burst comprises either 68 bits or 66
bits. Thus, the period from t1 to t2 (and also the period from t3
to t4) is either about 0.9167 ms or about 0.9444 ms.
The gap between time t2 and time t3, and the gap between time t4
and the following time t1, are present to give the base station 3
and the handset 11 time to switch between transmitting and
receiving modes, and also to allow for RF propagation delay of the
signals. The size of the gap between time t2 and time t3 is
determined by the handset 11. If the two bursts in the burst period
are each 66 bits long, the dset 11 will begin transmitting the
first bit of its burst 5.5 bit periods after the end of the last
bit of the received burst from the base station 3. If the two
bursts are each 68 bits, the handset waits for only 3.5 bit
periods. Assuming that there is no RF propagation delay of the
signals, the gap between time t4 and the following time t1 will be
1 bit period greater than the gap between time t2 and time t3. If
there is an RF propagation delay in the transmission of the
signals, the handset 11 will receive the transmission from the base
3 slightly later, and accordingly the transmission from the handset
11 will begin slightly later, and the gap between time t4 and the
following time t1 is accordingly reduced. The system can cope with
a cumulative propagation delay of 2 bit periods in each burst
period (typically up to 1 bit period in each direction), while
still allowing the base station 3 at least 2.5 bit periods to
switch from receiving to transmitting, even when 68-bit bursts are
being used. The base station 3 is assumed to be able to switch
faster than the handset 11 (which is allowed at least 3.5 bit
periods), to allow simpler and lower cost circuitry to be used in
the handset 11.
As will be explained later, communication between the base station
3 and the handset 11 may use more than one data structure for the
bursts. 68-bit bursts can only occur in one data structure, known
as multiplex 1, whereas a second data structure known as multiplex
2 always uses 66-bit bursts. During a change between communication
using multiplex 1 and communication using multiplex 2, it is
possible for there to be a brief period during which one part is
transmitting 68-bit multiplex 1 bursts while the other is
transmitting 66-bit multiplex 2 bursts. In this case, the gaps
between times t2 and t3 and between times t4 and the next t1 will
alter correspondingly.
In addition to radio signal (RF) propagation delays, there may be
signal delays through the circuitry of the base station 3 and the
handset 11. As these are internal to the devices, they can be
compensated for in the design of the devices. These delays are not
included in FIG. 2 which is concerned only with the timing of
signals at the aerials of the devices.
The digital bits are transmitted in each data burst by frequency
modulation, known as frequency shift keying (FSK), of the radio
carrier frequency.
The overall structure of a data burst is illustrated in FIG. 3. In
the gap between the time when one of the base station 3 and the
handset 11 stops transmitting and the time when the other starts
transmitting, the radio frequency will have zero amplitude. In
order to transmit the digital data of a burst, the radio frequency
signal must be transmitted at a suitable amplitude. In order to
avoid an amplitude modulation splash of interference on other
channels, the part which is about to transmit a burst begins to
transmit the radio frequency signal before the beginning of the
data burst, and slowly increases the amplitude of the signal. This
is the period from a to b in FIG. 3. The RF amplitude envelope is
shown at 15 in FIG. 3.
At instant b in FIG. 3, the first bit period of the data burst
begins. One binary bit of data is transmitted in each bit period,
by frequency modulation, with the transmitted frequency being
greater than the carrier frequency for the logical "1", and being
less than the carrier frequency for a logical "0". In order to
avoid a frequency modulation splash on other channels, the
transmitted frequency cannot be varied instantaneously, and
therefore it varies gradually as shown by the RF frequency envelope
17 in FIG. 3. Instant c in FIG. 3 represents the end of the first
bit period and the beginning of the second bit period. Instant d
represents the end of the last bit period.
Because of the delay dispersion introduced by some types of filter
used in the base station 3 and the handset 11, it is necessary to
maintain the amplitude of the RF signal at no more than 6 dB below
its amplitude during the data burst, for half a bit period until
instant e, to ensure that all data is correctly received and
processed at the receiving end. This half bit period is known as a
suffix, and is represented at reference numeral 19. Following the
end of the suffix 19 at instant e, the radio frequency is reduced
in amplitude gradually to avoid amplitude modulation splash, and
that ends the transmission by that part.
Data Structure of Bursts--MUX 1 and MUX 2
Communication between the base station 3 and the handset 11 takes
place over three logical channels. Since there is only one radio
channel between the two parts of the system, the three logical
channels are combined by time division multiplexing. At any given
time during an established link, the base station 3 and the handset
11 will be communicating with each other using bursts as described
above, having a pre-selected data structure for providing time
division multiplexing between the channels. In each of the
available data structures, each burst carries two of three logical
channels, multiplexed together. Each of these data structures is
known as a multiplex, abbreviated as "MUX".
Once a link has been set up, and the bursts between the base
station 3 and the handset 11 are carrying the information which the
user wishes to exchange with the person or device he is connected
to through the telecommunications network 1, the data structure of
bursts will be a form called multiplex 1 (or MUX 1). This is shown
in FIG. 4. Two forms of multiplex 1 are possible: multiplex 1.2 as
shown in FIG. 4a and multiplex 1.4 as shown in FIG. 4b.
In multiplex 1.2, each burst is 66 bits long. The first bit and the
last bit are defined as belonging to the D logical channel and the
central 64 bits are defined as belonging to the B logical channel.
The B channel carries the data which the user is transmitting or
receiving. In the normal case, where the handset 11 is being used
to hold a telephone conversation, the B channel will be carrying
digitally encoded speech sound data.
The D channel carries signalling data. This data may represent
various things, as will be described later, but most data carried
by the D channel can be assigned to one of two general types.
First, there is data communicated between the base station 3 and
the handset 11 purely for the purpose of establishing or
maintaining the radio link between the parts. This data includes
handshake signals, identification and authorisation codes which
enable one part to recognise the other and permit or refuse to
permit a communication link between them to be established, etc..
The second kind of data instructs the receiving part to take some
action or informs the receiving part that some action has occurred
at the transmitting part. For instance, if the user presses a key
on the handset 11, this fact will be transmitted to the base
station 3 by the D channel, and if the base station 3 instructs the
handset 11 to display a symbol or flash the display, this will be
transmitted in the D channel.
D channel data is transmitted in code words having a defined
format, which will be discussed later. Because each burst in
multiplex 1.2, as shown in FIG. 4a, contains only two bits of D
channel data, it requires a large number of bursts to transmit a
single code word of the D channel. In multiplex 1.4, as illustrated
in FIG. 4b, D channel data can be transmitted at twice the rate.
Each burst in this multiplex structure is 68 bits long. The first
two bits and the last two bits are defined as D channel bits, and
the central 64 bits are defined as B channel bits. It should be
noted that in both multiplex 1.2 and multiplex 1.4, each burst
carries the same number of B channel bits and the difference
between them is only in the number of D channel bits carried per
burst.
Although multiplex 1.4 is advantageous because it permits
transmission of the D channel at twice the rate permitted by
multiplex 1.2, this is achieved by making each burst in multiplex
1.4 two bits longer (i.e. 68 bits instead of 66 bits) than each
burst of multiplex 1.2. With reference to FIG. 2, this means that
when the two parts are communicating using multiplex 1.4, the gap
between times t2 and t3, during which each part can switch between
transmission mode and reception mode, is reduced by two bit periods
from 5.5 bit periods to 3.5 bit periods. The gap between time t4
and time t1 is similarly reduced by two bit periods. Thus,
communication using multiplex 1.4 can only take place if both the
base station 3 and the handset 11 are capable of switching between
transmission and reception modes in the reduced time available. A
device which cannot make this switch sufficiently quickly will only
be able to use multiplex 1.2.
All base stations 3 and handsets 11 in the preferred embodiment are
capable of communicating using multiplex 1.2, even if they are also
capable of communicating using multiplex 1.4. In any particular
radio link between a base station 3 and a handset 11, both parts
must use the same version of multiplex 1. During the initial
setting up of a link the two parts will communicate with a
different data structure, known as multiplex 2 (or MUX 2), before
switching to multiplex 1, and before switching to multiplex 1 the
two parts will perform an operation (sometimes known as a
"negotiation" operation) to determine which version of multiplex 1
will be used. If both parts can communicate using multiplex 1.4,
this will be done. If either or both of the parts can only use
multiplex 1.2, then this version of multiplex 1 must be adopted for
this radio link.
The structure of multiplex 2 is shown in FIG. 5. This data
structure is used in setting up a link before communication using
the B channel begins. In the multiplex 2 data structure, no B
channel is transmitted. Each multiplex 2 data burst is 66 bits
long. The first 16 bits and the last 16 bits are defined as
belonging to the D channel, and the central 34 bits are defined as
belonging to the third logical channel called the S channel. The
first 10 bits of the S channel are P bits, which form a preamble,
and simply comprise an alternation between "1" and "0". The
remaining 24 bits of the S channel are W bits, and define an S
channel synchronisation word.
The S channel is used to synchronise the two parts when a radio
link between them is being established. Two levels of
synchronisation are required. First, the parts must enter bit
synchronisation, so that the receiving part when decoding a
received signal divides the received signal into bits with the
correct timing. Second, the two parts must enter burst
synchronisation, so that one part transmits while the other is in
reception mode, and then the other transmits while the first is in
reception mode, to provide the alternating burst transmission
structure illustrated in FIG. 2.
When a link is being established in either direction the first
radio signals which a handset 11 receives from a base station 3
will be in the multiplex 2 format, and if the base station 3 is
initiating the creation of a link with a handset 11, the first
signals which the base station 3 receives from the handset 11 will
also be in the multiplex 2 format. (When the handset 11 is
initiating the link, it first transmits in a further data
structure, called multiplex 3 (or MUX 3), which will be described
later.) Therefore, multiplex 2 has been designed to permit rapid
synchronisation between the two parts.
Most base stations 3 and handsets 11 will contain automatic
frequency control or automatic gain control circuits, or both.
These circuits require an initial period, during which the radio
signal is received, to perform their control operations and
establish satisfactory radio reception by the part concerned. The
first 16 bits of D channel in the multiplex 2 structure provide
such a period of radio transmission, permitting the automatic gain
control and automatic frequency control circuits to settle before
the 34 bits of S channel are received.
The successive bit value reversals in the "1010 . . . " preamble
pattern of the P bits of the S channel provide a clear definition
of the timing of the bit periods, enabling the receiving part to
enter bit synchronisation with the transmitting part before the 24
bit synchronisation word of the S channel is received. The
synchronisation word has a predetermined pattern, which the
receiving part is searching for in the received data burst. When
this bit pattern is recognised, the receiving part knows that the
24 bits under consideration form the synchronisation word of the S
channel. Since the position of this word in the multiplex 2
structure is defined, the receiving part can then determine the
burst timing of the received multiplex 2 data burst, and therefore
burst synchronisation can be attained.
D channel data can vary, and it is possible that by chance 24
successive D channel bits could have the same pattern as an S
channel synchronisation word. If this was mis-identified as the S
channel synchronisation word, the receiving part would select an
incorrect burst timing. To prevent this, the D channel is split in
multiplex 2 into two 16 bit portions, so that the burst does not
contain 24 D channel bits as an unbroken string.
As described above with reference to FIG. 2, the base station 3
determines the 2 millisecond burst period by beginning the
transmission of successive data bursts from it at 2 millisecond
intervals. The handset 11 has to adapt its reception and
transmission timing to match the timing of the base station 3, and
thus create the time division duplex structure of FIG. 2. Thus, in
determining burst synchronisation timing the base station 3 acts as
a master and handset 11 acts as a slave. Since the base station 3
will typically contain more accurate clocks than handset 11, this
is convenient for maximising the accuracy of the burst mode timing.
Additionally, if a base station 3 is capable of communicating
simultaneously with different handsets 11 using different radio
channels, it will normally be impractical or even impossible for
the base station 3 to maintain two such links unless the
transmission and reception timing for the two links is
synchronised. Therefore, the timing of the handset 11 must be
slaved to the timing of the base station 3.
Link Initiation and MUX 3
If the base station 3 wishes to initiate a link, it will begin
transmitting in multiplex 2. If a handset 11 is switched on but is
not communicating in a link, it will be scanning radio channels
looking for a channel on which transmission is taking place. If the
handset 11 detects radio transmission, it will attempt to decode it
on the assumption that it is multiplex 2. The handset 11 performs
these operations asynchronously, and if the received signal is in
the multiplex 2 format the preamble portion and the synchronisation
word of the S channel will enable the handset 11 to obtain bit and
burst synchronisation with the transmitted signals. Once this
synchronisation has been obtained, the contents of the D channel
can be decoded and the process of link initiation can begin.
A problem may arise if a handset 11 wishes to initiate a
communications link with a base station 3. The operation of an idle
base station 3, i.e. one which is waiting for a call from a
handset, may be synchronised to the operation of an active base
station, i.e. one which is already in communication with a handset.
Thus the idle base station will only have a listening window during
which it can receive transmissions from a handset 11 during the
times in which the active base station is in the reception mode.
This restriction on the ability of an idle base station to listen
will almost inevitably be the case if the two base stations share a
common aerial, because RF power leaking from the transmitter of the
base station already in communication will probably swamp the
received power from a calling handset, so that the idle base
station would be unable to detect any handset during these periods,
even if it was in reception mode. Therefore, it must be assumed by
the handset 11 that a base station has a listening window of only
about 1 millisecond (72 bits) in every 2 millisecond (144 bit)
burst period.
A handset 11 which is not already in communication with a base
station 3 will not be synchronised with the base station 3, and
therefore it cannot send out a link request signal having a timing
synchronised to the listening periods of the base station 3 it is
trying to reach. Therefore, the handset 11 makes a link request by
transmitting asynchronously in a further data structure called
multiplex 3 (or MUX 3). The asynchronous nature of multiplex 3 is
shown in FIG. 6.
The lower line in FIG. 6 represents the timing of the operations of
the base station 3. The base station 3 will divide time into a
plurality of transmit periods 21, and will only be able to receive
signals from the handset 11 in between these transmit periods 21.
The top line in FIG. 6 shows the activity of a handset 11
transmitting in multiplex 3. Each multiplex transmission, indicated
at 23 in FIG. 6, is 720 bits long, and lasts for a period of 10
milliseconds. Successive multiplex 3 transmissions 23 are separated
by 288 bit periods, lasting 4 milliseconds, during which the
handset switches from transmission mode to reception mode and
listens for a reply in multiplex 2 from the base station 3. Thus,
the total burst period of multiplex 3 transmissions is 1008 bit
periods or 14 milliseconds. The multiplex 3 burst period is seven
times as long as the burst period for multiplex 1 and multiplex 2,
and the multiplex 3 transmission lasts for five multiplex 1 or
multiplex 2 burst periods.
As shown in FIG. 6, each multiplex 3 transmission 23 is divided
into five equal sub-multiplex sections. Each is 144 bits long, and
lasts for 2 milliseconds.
Thus, each sub-multiplex lasts for the same length of time as a
period of operation of the base station 3 including a transmission
period and a reception period.
FIG. 7 shows a portion of FIG. 6 on an enlarged scale. Each
sub-multiplex of multiplex 3 is in turn divided into four equal
repetition periods. The data transmitted in one sub-multiplex is
repeated in each repetition period. Each repetition period contains
36 bits and lasts for 0.5 milliseconds. Because the hand set 11 is
not synchronised with the base station 3, the timing of the signal
reception windows in the operation of the base station 3 with
respect to the multiplex 3 structure cannot be predicted. However,
each 36 bit repetition period of a sub-multiplex is sufficiently
short that the base station 3 must receive at least one complete
repetition period of a sub-multiplex in the period between
successive base station transmission periods 21. In FIG. 7, the
relative timings of the operation of the base station 3 and the
handset 11 are such that the third repetition period of each
sub-multiplex falls entirely within a base station reception
period, and will therefore be received by the base station 3.
Because each repetition period contains a complete copy of the data
being transmitted in one sub-multiplex period of a multiplex 3
burst, it is only necessary for the base station 3 to receive one
repetition period of each sub-multiplex to receive the data
transmitted by the handset 11. As shown in FIG. 6, the first four
sub-multiplex periods of multiplex 3 are defined as carrying the D
channel, while the final sub-multiplex period of multiplex 3 is
defined as carrying the S channel. This maximises the likelihood
that the base station 3 will receive and decode a repetition period
of the S channel.
Each repetition period of the S channel sub-multiplex of multiplex
3 contains an S channel synchronisation word. When the base station
3 detects this, it knows that there should be no multiplex 3
transmissions during the next two reception periods, and then in
successive reception periods it should receive four portions of D
channel and then an S channel portion including the synchronisation
word again. In this way, the base station 3 can be temporarily
synchronised with the multiplex 3 timing from the hand set 11,
without altering its own burst synchronisation for multiplex 1 and
multiplex 2 signals.
Following decoding of the multiplex 3 signals received from the
handset 11, the base station can reply in multiplex 2 during the
288 bit space between successive multiplex 3 transmissions. This
space is sufficiently long that it can be guaranteed to include at
least one complete base station transmit period 21, as can be seen
in FIG. 6. Once the handset 11 receives a reply in multiplex 2 from
the base station 3, it ceases to transmit in multiplex 3 and
transmits instead in multiplex 2, synchronised to the timing of the
base station 3.
The arrangement of data in the multiplex 3 structure is shown in
more detail in FIGS. 8 and 9. In FIG. 8 each line represents one
sub-multiplex period. The first four lines represent the four D
channel sub-multiplex periods, and the fifth line represents the S
channel sub-multiplex period. The sixth and seventh lines represent
the period (equal to two sub-multiplex periods) which follow one
multiplex 3 transmission before the next multiplex 3 transmission.
The handset 11 listens for a reply in multiplex 2 during this
period.
The five lines of the multiplex 3 transmission are each divided
into the four repetition periods. In the first four lines, each
repetition period contains 36 D channel bits. Successive repetition
periods of the same sub-multiplex are identical, but successive
sub-multiplexes carry different D channel information. Each
repetition period of the fifth line in FIG. 8, representing the
fifth sub-multiplex, contains 36 bits of S channel. The S channel
data in each repetition period is identical. Thus, the entirety of
the data transmitted in a multiplex 3 burst is contained in a
single column in FIG. 8.
FIG. 9 shows in more detail the arrangement of data in a single
column of FIG. 8. In the D channel, only some of the bits carry
useful data. Each 36 bit repetition period begins with 6 P bits
forming a preamble portion of alternating logic "1"and "0". Then
there are 10 data carrying bits, then a further 8 P bits, then a
further 10 data carrying bits and finally 2 further P bits. In this
way, each repetition period contains 20 data carrying bits of the D
channel, divided into two stretches of 10 bits separated by 8 bits
of preamble. The data carrying bits can adopt any values, depending
on the data being transmitted in the D channel, and therefore it is
theoretically possible that the pattern of data bits in the D
channel may be identical to or may closely resemble the
synchronisation word of the S channel. If the D channel data was
transmitted continuously and this pattern of bits arose in it by
chance, the base station 3 would mis-identify the received D
channel repetition period as belonging to the S channel, and
therefore it would decode multiplex 3 incorrectly. By splitting the
data carrying bits of the D channel into sections of 10 bits,
separated by 8 bits of preamble, the D channel in multiplex 3 can
never contain a pattern of successive bits resembling the pattern
of the S channel synchronisation word.
As shown in FIG. 9, each repetition period of the S channel
sub-multiplex of multiplex 3 begins with a preamble of 12 P
bits,alternating between logic "1"and logic "0". This is followed
by 24 W bits making the S channel sychronisation word. This
arrangement of the S channel in multiplex 3 maximises the
opportunity for the receiving base station 3 to obtain bit
synchronisation with the multiplex 3 signal before it receives the
synchronisation word.
S Channel Structure
The structure of the S channel is very simple. It consists of 10
bits of preamble followed by 24 bits of synchronisation word in
multiplex 2, as shown in FIG. 5, or 12 bits of preamble followed by
a 24 bit synchronisation word in multiplex 3, as shown in FIG. 9.
The preamble is always made up of alternating logic "1"and "0".
In the system of the preferred embodiment, there are four possible
S channel synchronisation words. Two of these are used only by base
stations 3, and the other two are used only by handsets 11. When a
part wishes to initiate a link, it uses an S channel
synchronisation word known as a channel marker, abbreviated CHM.
When the other part receives the signals, it replies using a normal
synchronisation word, abbreviated SYNC. When the link between the
two parts is established, following reception of this reply, the
first part changes the synchronisation word in the S channel in its
transmissions from CHM to its version of SYNC. The version of CHM
for the base station 3 is called the fixed part channel marker,
abbreviated CHMF, and the channel marker for the hand set 11 is
called the portable part channel marker, abbreviated CHMP.
Similarly, the base station 3 version of SYNC is referred to as
SYNCF and the handset 11 version of SYNC is referred to as SYNCP.
CHMF and CHMP are bit inverses of each other, and SYNCF and SYNCP
are bit inverses of each other.
Thus, if a base station 3 wishes to initiate a link it will
transmit a link request using multiplex 2, with CHMF as the S
channel synchronisation word. A handset 11 receiving this
transmission will reply in multiplex 2, using SYNCP as the S
channel synchronisation word. Once the link is established the base
station 3 will change its multiplex 2 bursts to use SYNCF as the S
channel synchronisation word instead of CHMF.
If a handset 11 wishes to initiate a link, it will transmit a link
request in multiplex 3, using CHMP as the S channel synchronisation
word. When this is detected by a base station 3, it will reply in
multiplex 2 using SYNCF as the S channel synchronisation word. Once
the link is established, the handset 11 will change its
transmissions to multiplex 2, using SYNCP as the synchronisation
word.
When a base station 3 or a handset 11 is scanning channels to
determine whether another part is requesting a link with it, it
will only react to a CHM synchronisation word, since this indicates
that there is another part wishing to set up a link. If a SYNC
synchronisation word is detected, this indicates that the channel
contains a link which has already been set up, and therefore the
part which is scanning channels should not react.
Base stations 3 are arranged to be capable of recognising CHMP and
SYNCP, i.e. the S channel synchronisation words transmitted by a
handset 11, but are not capable of recognising CHMF or SYNCF.
Therefore, a base station 3 will never decode and respond to a
multiplex 2 transmission from another base station, even if it is
received. Similarly, handsets 11 can only recognise CHMF and SYNCF,
and not CHMP and SYNCP, so that handsets cannot recognise multiplex
2 and multiplex 3 transmissions from each other and therefore
handsets can never initiate links directly between themselves but
only with base stations.
Hardware
Following the above description of the manner in which the base
station 3 and the handset 11 exchange signals, the base station 3
and the handset 11 themselves will now be described.
FIG. 10 shows an example of a handset 11. It has an aerial 25 for
transmitting and receiving signals in the radio link with the base
station 3. The aerial 25 may alternatively be provided out of sight
inside the casing of the handset 11. The handset 11 has a
microphone 27 and a speaker 29 for use in telephone voice
communication, and has a keypad 31 for controlling its operations.
The bottom four rows of the keypad provide a convention telephone
keypad, allowing numbers to be dialled etc., including 0 to 9
numeric keys, a "hash" key and a "*" key. The keys 33 of the top
row allow the user to control the radio link with a base station 3.
By pressing an appropriate key 33 the user can accept a call to the
handset 11 from a base station 3, or request access to the
telecommunications network 1 through a nearby base station 3.
The handset 11 may be provided with other conventional features,
such as memories for pre-storing telephone numbers, and a display.
An example of a handset 11 having a display 35 is shown in FIG. 11.
Apart from the presence of the display 35, this handset is the same
as the handset of FIG. 10.
FIG. 12 is a schematic diagram of the handset construction. The
handset is controlled by a control circuit 37, which is connected
to the aerial 25, the microphone 27, the speaker 29 and a
keypad/display unit 39 which provides the keypad 31, and also the
display 35 if it is provided. One or more batteries 41 provide
power to the control circuit 37 and the key pad/display unit
39.
FIG. 13 shows an example of a base station 3. This has an aerial 43
for communication with cordless telephone handsets 11, and is also
connected through a conventional telephone connection 45 to the
telecommunications network 1 and though a power connection 47 to a
conventional electricity power source.
The base station 3 is also provided with a conventional wired
telephone handset 49, a display 51 and a keypad 53, which enable
the base station 3 to be used as a conventional telephone connected
to the telecommunications network 1 and also as an intercom station
which can communicate with a cordless telephone handset 11 without
involving the telecommunications network 1. FIG. 14 provides a
schematic view of the construction of the base station 3. It is
controlled by a base station control circuit 55, which is connected
to the wired handset 49, to the telephone connection 45 and to a
keypad/display unit 57 which provides the keypad 53 and display 51
in a conventional manner. A power supply circuit 59 receives
electricity from the power connection 47, and supplies power to the
control circuit 55 and the key pad/display unit 57. The power
supply circuit 59 may include power storage means such as batteries
or capacitors, enabling the base station to operate for a limited
period even when disconnected from an external electricity
supply.
The base station control circuit 55 includes a switching means 61,
connected to the wired handset 49, the telephone connection 45 and
the aerial 43. In one state of the switching means 61, it connects
the wired handset 49 to the telephone connection 45 to permit
normal telephone operation without the speech signals being
processed by the control circuit 55 in the manner required for
transmission over the radio link. In another state, the switching
means 61 connects the wired handset 49 and the aerial 43 to the
remainder of the control circuit 55 so that speech signals pass
between the wired handset 49 and the aerial 43 having been
processed as required to permit communication over the radio link
with a cordless handset 11. In a third state the switching means 61
connects the telephone connection 45 to the remainder of the
control circuit 55 in place of the wired handset 49, enabling the
base station 3 to act simply as a base for the radio link between
the remote handset 11 and the telecommunications network 1.
Where the functions of the wired handset 49 are not required, the
base station 3 may not include any of the wired handset 49, the
display 51, the keypad 53 the keypad/display unit 57 and the
switching means 61. The base station control circuit 55 would be
permanently connected to both the aerial 43 and the telephone
connection 45.
As with the cordless handset 11, the base station aerial 43 may be
contained within the casing of the base station 3.
FIG. 15 is a block diagram of the circuitry of the handset 11.
Speech sounds received by the microphone 27 are converted into an
electrical signal which is provided to a speech encoder 63. The
speech encoder 63 includes an analogue-to-digital converter which
converts the analogue electrical signal from the microphone 27 into
8-bit digital signals with a sampling rate of 8 kHz. This results
in a total bit rate of 64 kbit per second. The analogue-to-digital
conversion is non-linear, and has the effect of performing Pulse
Code Modulation (PCM) on the input.
The 8-bit data words are then compressed to 4-bit data words,
thereby reducing the bit rate of the digital data to 32 kbit per
second. The compression is done by Adaptive Differential Pulse Code
Modulation (ADPCM). In this coding system, each 4-bit word
represents the change in value between the successive samples,
rather than the absolute sample values themselves. This is an
effective data compression technique for signals which change
relatively slowly, such as speech signals. The 32 kbit per second
data stream provides the contents of the B channel, and is provided
to a B channel input to a programmable multiplexer 65 as 4-bit
parallel words at 8 kword per second.
The speech encoder 63 may also reverse the values of some bits in
the B channel data, according to a predetermined pattern, in order
to increase the probability of changes of bit value between
adjacent bits of the data. This is for the benefit of the radio
transmitting and receiving systems, which may work better if the
data value of transmitted and received signal bits changes
frequently.
The programmable multiplexer 65 also receives D channel data and S
channel data at respective inputs. While the handset 11 is
operating in multiplex 1, the programmable multiplexer stores the
continuously received 32 kbit per second data stream from the
speech encoder 63. The programmable multiplexer 65 outputs data in
bursts, in accordance with the burst mode operation of the radio
link, at 72 kbit per second in accordance with the data rate of the
radio link. Thus, once in each 2 ms burst period, the programmable
multiplexer will output 64 bits of B channel data previously
received from speech encoder 63 and stored, and will sandwich the B
channel data between 2 or 4 bits of D channel data to form the
multiplex 1.2 or multiplex 1.4 data streams.
The data stream burst from the programmable multiplexer 65 is
provided to a transmitter 67, which modulates the radio carrier
frequency, received from a local oscillator 69, in accordance with
the received data stream. The resulting radio frequency burst is
provided to the aerial 25 via a transmit/receive switch 71. The
transmit/receive switch 71 connects the transmitter 67 to the
aerial 25 during the transmit part of each burst period and
connects the aerial 25 to a radio receiver 73 during the receive
part of each burst period.
During the receive part of each burst period, the receiver 73
demodulates the received signal from the aerial 25, using a carrier
frequency signal from the local oscillator 69. The demodulated 72
kbit per second data stream burst is provided by the receiver 73 to
a programmable demultiplexer 75.
The programmable demultiplexer 75 allocates the received data bits
between the B channel, the S channel and the D channel in
accordance with the multiplex structure in which the handset 11 is
currently operating. When the handset is operating in multiplex 1,
the 64 B channel bits received in each data burst are stored in the
programmable demultiplexer 75, and are then output to a speech
decoder 77 as a continuous stream of 4-bit parallel words at 8
kword per second.
The speech decoder 77 repeats the pattern of bit reversals applied
to the B channel data by the encoder in the base station 3, to
obtain the correct data values, and then performs the inverse of
the ADPCM algorithm used to encode the speech data, so as to obtain
8-bit Pulse Code Modulated words at a rate of 8 kword per second.
The speech decoder then converts this digital data to analogue data
in a PCM digital-to-analogue converter, and provides the output
analogue signal to the speaker 29. The speaker 29 converts the
analogue electric signal to sound to be heard by the user.
During multiplex 1 operation, the speech encoder 63 provides B
channel data to the programmable multiplexer 65 at 32 kbit per
second. Thus in each 2 ms burst period, the programmable
multiplexer 65 receives 64 B channel bits. Since each multiplex 1
burst carries 64 B channel bits, the radio link carries the B
channel at an effective average bit rate equal to the bit rate
provided by the speech encoder 63. Similarly, the effective average
bit rate of received B channel data matches the bit rate of the
continuous data transmission from the programmable demultiplexer 75
to the speech decoder 77. Thus, there is an effective continuous
bidirectional B channel communication, in spite of the time
division duplex burst mode nature of the radio link.
As will be well known to those skilled in the art, the speech
encoder 63 and the speech decoder 77 can conveniently be provided
by a single circuitry unit known as a coder/decoder or codec.
The operation of the handset 11 is controlled by a system
controller 79, and the timing of operations is controlled, in order
to ensure burst synchronisation, in response to signals from an S
channel controller 81. The system controller 79 is typically a
microprocessor-based or microcomputer-based control system,
including a processor, a program memory and a random access memory.
The S channel controller 81 may be implemented as a separate
microprocessor or may be implemented in software for the same
processor as the system controller 79. However, in view of the
simple nature of the operations carried out by the S channel
controller 81, and the need for high speed in its operations, it is
preferably implemented as dedicated hardware.
The system controller 79 sends control signals to the programmable
multiplexer 65 and the programmable demultiplexer 75, to instruct
them which multiplex structure to adopt, and also to give them
timing signals so that they are properly synchronised with the
radio link burst structure. The programmable multiplexer 65 and
programmable demultiplexer 75 may also send signals to the system
controller 79 to inform it if a buffer used to store the data
signals in the multiplexer or demultiplexer is approaching overflow
or is empty.
Control signals from the system controller 79 control the
transmit/receive switch 71, so that it connects the transmitter 67
and the receiver 73 to the aerial 25 alternately with the correct
timing.
The system controller 79 selects the radio channel on which the
handset 11 is operating at any given moment, and instructs the
local oscillator 69 to generate a signal for the transmitter 67 and
receiver 73 at the appropriate frequency. In a system intended for
use in Great Britain in accordance with the regulations issued by
the Department of Trade and Industry, the handset 11 may operate on
any one of forty channels having carrier frequencies at 100 kHz
spacings in the range 864.15 MHz to 868.05 MHz. The system
controller 79 will inform the local oscillator 69 which channel has
been selected, and the local oscillator 69 will inform the system
controller 79 when its output signal has reached the selected
frequency.
The system controller 79 may also send control signals to the
speech encoder 63 and speech decoder 77 to mute the B channel at
certain times. It is advantageous to mute the B channel during link
set up and also if it becomes necessary to re-establish a link
during conversation, in order to prevent the user from receiving
unpleasant noises at these times.
The system controller 79 also controls the D channel. It receives
incoming D channel data from the programmable D multiplexer 75 and
provides outgoing D channel data for transmission to the
programmable multiplexer 65. Some received D channel data is used
purely to control the operation of the system controller 79, and
some transmitted D channel data is generated within the system
controller 79. Such data includes transmitted and received
handshake signals and various identification signals which are
exchanged between the handset 11 and a base station 3 during the
establishment of a radio link. However, other types of transmitted
D channel data will result from actions taken by the user, and
other types of received D channel data must be passed on to the
user. For this reason, the system controller 79 also has a control
signal connection with the keypad and display unit 39.
When a user is initiating a telephone call from the handset 11 the
telephone number to be dialled will be entered through the keypad
31. The key depressions will be notified by the keypad/display unit
39 to the system controller 79, which will encode them for
transmission in the D channel. In this way, the base station 3 is
informed of the telephone number dialled by the user, and can
transmit the appropriate dialling signal to the telecommunications
network 1.
If a base station 3 initiates a radio link with the handset 11,
because a telephone call has been received, the user must be
alerted to the presence of the incoming call. To do this, the
system controller may control a tone caller (not separately
illustrated) to give an audible notification. Additionally, the
system controller 79 may instruct the keypad/display unit to
provide a visual indication e.g. with a light. If the user wishes
to accept the call, this can be done by pressing a "line" key 33.
This is notified by the keypad/display unit 39 to the system
controller 79, which in turn notifies the base station 3 through
the D channel.
If the handset 11 includes a display 35, information may be
displayed on it in accordance with instructions from the system
controller 79, both before a user has accepted a call and during a
conversation. The data to be displayed will typically have been
received by the system controller 79 from the base station 3 over
the D channel.
The S channel controller 81 receives S channel data from the
programmable demultiplexer 75, and provides S channel data for
transmission to the programmable multiplexer 65. When the handset
11 is idle, and scanning radio channels to see whether a base
station 3 is calling it, the system controller 79 controls the
transmit/receive switch 71 to connect the aerial 25 permanently to
the receiver 73. When a radio signal is received, bit
synchronisation is achieved in the programmable demultiplexer 75,
but the S channel controller 81 is responsible for recognising the
S channel synchronisation word and enabling burst synchronisation.
Until burst synchronisation is achieved all received data is
treated as potentially belonging to the S channel and is passed by
the programmable demultiplexer 75 to the S channel controller 81.
The S channel controller 81 searches the incoming data for the S
channel synchronisation word CHMF, which is used by a base station
3 when it is wishing to set up a link.
When the S channel controller 81 recognises CHMF, it notifies the
system controller 79 that the base station channel marker has been
received and also provides a frame clock synchronised to the timing
of the received burst. The system controller 79 uses the burst
timing information from the S channel controller 81 to control the
timing of the operation of the programmable demultiplexer 75, so
that further transmissions from the base station 3 are decoded into
the correct logical channels. At this stage, the programmable
demultiplexer 75 will be operating in multiplex 2. The programmable
demultiplexer 75 divides the received data between the S and D
channels according to the multiplex 2 data structure. Provided that
the S channel data sent to the S channel controller 81 continues to
include the synchronisation word CHMF, the S channel controller
will continue to confirm the burst synchronisation to the system
controller 79.
The system controller 79 decodes the data received on the D
channel. If this leads it to reply to the received transmission, it
will instruct the programmable multiplexer 65 to begin operations
in multiplex 2, with the appropriate burst timing, and will control
the transmit/receive switch 71 to alternate between connecting the
aerial 25 to the receiver 73 and the transmitter 67. At the same
time, the system controller 79 will instruct the S channel
controller 81 to provide the SYNCP synchronisation word to the
programmable multiplexer 65 as S channel input.
If the user of the handset 11 wishes to initiate a call, and
therefore presses one of the keys 33 on the keypad 31, the
keypad/display unit 39 will notify this to the system controller
79. The system controller 79 searches through the RF channels, by
changing the frequency of the local oscillator 69, until an empty
channel is found. An empty channel is defined as one on which the
received radio frequency energy is below a threshold value. If the
received radio frequency energy is above the threshold value on all
channels, the channel on which the least radio frequency energy is
received is defined as an empty channel.
The system controller 79 then instructs the programmable
multiplexer 65 to operate in multiplex 3, and instructs the S
channel controller 81 to provide the portable part channel marker
CHMP to the programmable multiplexer 65 as the S channel
synchronisation word. The transmit/receive switch 71 is controlled
to connect the aerial 25 to the transmitter 67 and the receiver 63
in the pattern required for multiplex 3 operation, and the system
controller 79 ensures that the switching of the transmit/receive
switch 71 is synchronised with the multiplex 3 operation of the
programmable multiplexer 65.
During receive periods, the programmable demultiplexer 75 passes
any received data to the S channel controller 81. The received data
should include SYNCF. When this synchronisation word is identified,
the S channel controller 81 provides the system controller 79 with
the burst timing of the received signals. The system controller 79
then instructs the programmable demultiplexer 75 to decode received
data as multiplex 2, in accordance with the received burst timing.
Once the received channel data has been decoded by the system
controller 79, it will instruct the programmable multiplexer 65 to
switch to multiplex 2 with timing synchronised with the burst
timing information from the S channel controller 81. It will also
change the timing of the control signals to the transmit/receive
switch 71 appropriately, and will instruct the S channel controller
81 to provide SYNCP to the programmable multiplexer 65 in place of
CHMP.
A modified handset 11 may be used to communicate digital data e.g.
to and from a portable personal computer or computer terminal,
rather than communicate speech. In this case, the microphone 27,
speaker 29 and keypad/display unit 39 are replaced by an interface
to the computer or terminal, and modification of the speech encoder
63 and speech decoder 77 may be required. In particular, the
computer or terminal will normally provide and receive digital
data, so that the analogue-to-digital converter of the speech
encoder 63 and the digital-to-analogue converter of the speech
decoder 77 will not be required. Additionally, computer data is not
normally suitable for data compression using Adaptive Differential
Pulse Code Modulation. Therefore, the data coding and decoding
operations of the speech encoder 63 and speech decoder 77 may need
to be modified. Alternatively, if the computer or terminal can be
set to operate at the data rate of 32 kbit per second, the encoder
and decoder can be omitted entirely.
FIG. 16 shows a schematic block diagram of a base station 3. This
is a simple base station, not including a wired handset 49, a
display 51 and a key pad 53.
As can be seen, the general construction of the base station
control circuit 55 is similar to that of the handset control
circuit 37. The programmable multiplexer 85, the transmitter 87,
the local oscillator 89, the transmit/receive switch 91, the
receiver 93 and the programmable demultiplexer 95 are substantially
identical with the corresponding parts in the transmitter 11. The S
channel controller 101 of the base station 3 is also similar to the
S channel controller 81 of the transmitter 11, except that the base
station S channel controller 101 is designed to recognise CHMP and
SYNCP in the incoming S channel data, and to provide CHMF and SYNCF
to the programmable multiplexer for transmission, instead of the
other way round.
The operation of the system controller 99 is generally similar to
the operation of the system controller 79 of the transmitter 11,
but there are some differences. First, when the base station 3 is
trying to set up a radio link with a handset 11, it transmits in
multiplex 2 rather than multiplex 3, and so the instructions to the
programmable multiplexer 85 and the timing signals to the
transmit/receive switch 91 in these circumstances is different.
Similarly, when the base station 3 is scanning the radio channels
to detect whether a handset 11 is calling it, it expects the
handset 11 to be calling using multiplex 3. Accordingly, once the S
channel controller 101 has notified the system controller 99 that
the handset channel marker CHMP has been received, the system
controller 99 will instruct the programmable demultiplexer 95 to
treat incoming signals as having the data structure of multiplex 3.
Once the base station 3 has sent a reply to a received multiplex 3
signal, it expects the handset 11 to change to multiplex 2, and
therefore it will instruct the programmable demultiplexer 95
accordingly at this time.
Since the burst timing of the handset 11 is slaved to the timing of
the base station 3, except during multiplex 3 transmissions, the
timing information received by the system controller 99 from the S
channel controller 101 is not used to control the timing of the
operations of the programmable multiplexer 85. The timing of the
programmable multiplexer 85 and the transmit/receive switch 91 is
determined by an internal clock of the system controller 99.
However the programmable demultiplexer 95 is controlled in
accordance with the received burst timing, both to enable correct
decoding of multiplex 3 transmissions from a handset 11 and to
compensate for the effect of RF transmission delays on
transmissions from the handset 11. The system controller 99 may
also use synchronisation timing information from the S channel
controller 101 as one way of determining that a communication link
with a handset 11 has broken down through loss of burst
synchronization.
A second area in which the operations of the system controller 99
in the base station 3 are different from the operation of the
system controller 79 in the hand set 11 is in its processing of D
channel data. The signalling data received by the base station 3
from the telecommunications network 1 will be different from the
signalling data input to the handset 11 by a user, and there will
be corresponding differences in the D channel data received by each
part over the radio link. Accordingly, the programming of the
system controller 99 in the details of its handling of D channel
data will be different.
Also, the actions taken by the base station 3 during link
initiation are different from the actions of the handset 11, as
will be described in detail later, and so the respective system
controllers 99, 79 will be programmed differently in this
respect.
The base station control circuit 55 includes a line interface 103,
to which the telephone connection 45 is connected. The line
interface 103 replaces the microphone 27, the loud speaker 29 and
the keyboard/display unit 39 in the arrangement of the control
circuit. Signalling data output by the system controller 99,
typically in response to received D channel data, is conditioned by
the line interface 103 and placed on the telephone connection 45.
Signals received from the telecommunciations network 1 over the
telephone connection 45 are similarly interpreted by the line
interface 103 and provided to the system controller 99 as required.
The line interface 103 also receives the decoded B channel data
stream from the decoder 97 and places this on the telephone
connection 45, and receives the speech or other communication
signals from the telephone connection 45 and provides these to the
encoder 83.
The manner of operation of the line interface 103 will be chosen in
accordance with the nature of the telecommunications network 1 to
which the base station is connected. In particular, if the base
station 3 3 is connected to a conventional PSTN, the line interface
103 will send and receive analogue signals over the telephone
connection 45, whereas if the base station 3 is connected to an
ISDN, the line interface 103 will normally be required to send and
receive 64 kbit per second pulse mode modulated signals.
In order to allow the base station 3 to communicate with various
different types of handset 11, the encoder 83 and decoder 97 are
enabled to carry out various encoding and decoding operations. They
may be able to use a plurality of different adaptive differential
pulse code modulation algorithms. They may also be able to use a
digital data processing algorithm or to pass signals through
unaltered to enable the base station 3 to be usable with portable
computer and computer terminal type handsets 11 as mentioned above.
During the link set up procedure, while the base station 3 and the
handset 11 are communicating in multiplex 2, the handset 11 can
indicate through the D channel the type of coding and decoding it
requires, and the system controller 99 of the base station 3 will
then control the encoder 83 and decoder 97 to operate accordingly
once multiplex 1 transmissions have begun.
FIG. 17 illustrates in block form the programmable multiplexers 65,
85. B channel data is output by the encoder 63, 83 as 4 bit
parallel words at a rate of 8 kword per second. The 4 bits of each
word are received in parallel, and they are stored in a B channel
elastic store 105 in the programmable multiplexer under the control
of an 8 kHz read clock which is synchronised with the operations of
the encoder 63, 83.
The D channel data is provided in 8 bit parallel words by the
system controller 99. This data may be provided intermittently, and
the average rate of D channel data will vary depending on the
multiplex data structure being used. The D channel data is received
by a D channel elastic store 107, and is clocked into the elastic
store by a clock signal provided by the system controller 99.
In a similar manner, the S channel data is provided to an S channel
elastic store 109, and is clocked into it by a clock signal
synchronised with the operation of the S channel controller 81,
101. It would be possible to eliminate the S channel elastic store
109, and provide the S channel data from the S channel controller
81, 101 to the programmable multiplexer 65, 85 with the correct
timing for it to be slotted into the data burst. However, this
would require the operations of the S channel controller to be
precisely synchronised with the operations of the programmable
multiplexer, and the data structure of the burst would be disrupted
if there was any variation in bit or burst synchronisation between
them. The use of the S channel elastic store 109 enables the
programmable multiplexer to ensure that the S channel data is
placed in the data burst with the correct timing, regardless of any
slight differences in timing of the S channel controller.
Additionally, since the contents of the S channel will typically be
the same from one data burst to the next, the S channel data can be
stored in the S channel elastic store 109 and read out repeatedly,
and the S channel controller only needs to provide new S channel
data to the programmable multiplexer if the S channel
synchronisation word is being changed. The S channel preamble may
be stored permanently in the S channel elastic store 109.
The multiplexing operation of the programmable multiplexer is
controlled by a multiplex controller 111. This receives signals
from the system controller 79, 99, informing it which multiplex
structure is being used, and also giving it the correct burst
timing. The multiplex controller 111 may also receive a clock
signal from the system controller, or alternatively it may have an
internal clock generator, which is synchronised to the burst timing
signal from the system controller.
Signals are read out from the B channel elastic store 105, the D
channel elastic store 107 and the S channel elastic store 109 under
the control of the multiplex controller 111, and the multiplexing
of the signals takes place in a signal combiner 113. The signal
combiner 113 receives input from each of the elastic stores 105,
107, 109, and it selects the signal received at one of these inputs
to be passed on to the output under the control of an input select
signal provided to the signal combiner 113 by the multiplex
controller 111. Simultaneously, the multiplex controller 111
provides control signals to the elastic stores 105, 107, 109, so
that each elastic store reads out one or more bits of its contents
as a serial bit stream, when its input to the signal combiner 113
is connected to the output. The multiplex controller 111 provides a
72 kHz clock to each of the elastic stores 105, 107, 109, so that
the signals are read out from these stores at the correct bit rate
for the data burst being assembled by the programmable
multiplexer.
The B channel elastic store 105 and the D channel elastic store 107
provide control signals to the multiplex controller 111, indicating
the amount of data currently stored in the stores. The multiplex
controller 111 notifies the system controller 79, 99, if either of
these stores is about to overflow or alternatively if either of
these stores contains no data when the multiplex structure being
transmitted requires data to be sent in the relevant channel.
FIG. 18 is a block diagram of the programmable demultiplexers 75,
95. The incoming demodulated signal from the receiver 73, 93, is
provided first to a re-timing unit 115. This continuously monitors
changes in the level of the signal provided by the receiver, in
order to maintain bit synchronisation of the demultiplexer with the
received signal. The incoming data signal is then passed to a
signal separator 117, while data on the received signal bit timing
is provided to a demultiplex controller 119. The demultiplex
controller provides a data distribution signal to the signal
separator 117, which controls the manner in which the signal
separator 117 distributes the data received at its input between
three outputs, one for each of the B channel, the S channel and the
D channel.
The demultiplex controller 119 receives control signals from the
system controller 79, 99, informing it which multiplex structure
the incoming data should be treated as having. If the handset 11 or
base station 3 is scanning the radio channels looking for a signal
indicating that another part wishes to set up a link with it, the
demultiplex controller 119 will direct the signal separator 117 to
pass all data received by the programmable demultiplexer to the S
channel. The S channel data is provided directly to the S channel
controller 81, 101. No elastic store is used for the S channel in
the programmable demultiplexer, as delays to the S channel data in
such an elastic store could prevent the S channel controller from
detecting the burst synchronisation of the received signal
correctly.
Once the burst synchronisation of the incoming signal has been
detected, the system controller 81, 101 will instruct the
demultiplex controller 119 to treat incoming data as having a
specified multiplex structure, and it will also provide the
demultiplex controller 119 with burst synchronisation timing. In
accordance with the instructions from the system controller, the
demultiplex controller 119 will control the signal separator 117 to
distribute the incoming data between the three channels.
The B channel data is provided to a B channel elastic store 121 and
the D channel data is provided to a D channel elastic store 123. In
all cases, the received data will be in the form of a serial bit
stream at 72 kbit per second. The data is clocked into the elastic
stores 121, 123 in accordance with a 72 kHz clock signal provided
to the stores by the demultiplex controller 119. The received
signal bit timing information provided by the re-timing unit 115 to
the demultiplex controller 119 is used by the demultiplex
controller 119 to ensure that the 72 kHz clock is correctly
synchronised with the data received by the elastic stores 121,
123.
The demultiplex controller 119 controls the operation of the
elastic stores 121, 123 so that they only store data while data for
that store is being provided by the signal separator 117. The
stores provide the demultiplex controller 119 with information on
how much data they contain, and the demultiplex controller 119
warns the system controller 79, 99 if either store is empty or is
about to overflow.
B channel data is read out of the B channel elastic store 121 as 4
bit parallel words, which are passed to the decoder 77, 97. The 4
bit words are read out at 8 kword per second in accordance with an
8 kHz clock provided to the B channel elastic store 121 and
synchronised with the operations of the decoder.
The D channel information is read out of the D channel elastic
store 123 as required by the system controller 79, 99, as 8 bit
wide parallel words. This operation is performed in accordance with
a read clock signal provided to the D channel elastic store 123 by
the system controller.
FIG. 19 is a schematic diagram of the system controller 79, 99. The
system controller comprises a microprocessor 125, having a clock
device 127 connected to it in the conventional manner. A bus 129
for address, data and control signals connects the microprocessor
125 to a random access memory 131 and a read only memory 133. The
random access memory 131 provides working memory for the
microprocessor 125, and the read only memory 133 contains the
program for the microprocessor 125.
It will normally be necessary for handsets 11, at least, to perform
a registration operation before use, in order to acquire a code
word which permits access to a base station 3 or a group of base
stations 3, or to one of a plurality of facilities offered by a
base station 3. In order to permit such a code word to be stored
safely, and retained even if power is removed from the device (e.g.
when changing the batteries in a handset 11), it is preferred that
an electrically alterable read only memory (EAROM) 134 is provided
in which such a code word may be stored. In an alternative, either
the random access memory 131 or the read only memory 133 is an
EAROM, and no separate EAROM 134 is provided, but this will
normally be a more expensive implementation.
Other parts of the device are connected to the system controller as
shown in FIGS. 15 and 16. As will be well known to those skilled in
the microprocessor art, these other devices can either be connected
as peripheral devices, or as memory mapped devices. Memory mapped
devices are connected directly to the bus 129. Peripheral devices
are connected to an input/output interface 135, which is in turn
connected to the bus 129.
While the handset 11 or the base station 3 is active but not
connected in a radio link, it will be scanning the radio channels
to determine whether another device is seeking to initiate a radio
link. At the same time, the system controller 79, 99 must respond
if a user presses a button requesting that a radio link is set up,
in the case of a handset 11, or a telephone ringing signal is
received from the telecommunications network 1, in the case of a
base station 3. This may be done by programming the system
controller so that it polls the keypad/display unit 39, in the case
of a handset 11, or the line interface 103 in the case of a base
station 3, to determine whether a signal has been received which
the system controller must respond to. Alternatively, the
keypad/display unit 39 and the link interface 103 may be connected
through control lines of the bus 129 to interrupt inputs to the
microprocessor 125, so that the channel scanning operation of the
system controller 79, 99 is interrupted if a signal is received
requiring the device to initiate a link itself. These alternatives
in the construction and programming of microprocessor controlled
devices will be well understood by those skilled in the art.
FIG. 20 is a schematic block diagram of the S channel controller
81, 101. The S channel controller has a CHM synchronisation word
recogniser 137 and a SYNC synchronisation recogniser 139. These
continuously compare the 24 most recently received bits of data
provided to the S channel controller from the programmable
demultiplexer 75, 95 with stored representations of the
synchronisation words, and provide respective "CHM recognised" and
"SYNC recognised" signals whenever a match between the S channel
input and the stored synchronisation words is obtained. The
recognisers 137, 139 may each be implemented by a 24 bit serial
input shift register, having parallel outputs to first inputs of
respective bit recognisers, the second inputs of which are
connected to hard-wired representations of the bits of the
synchronisation word.
For each synchronisation word, the version for the handset 11 is
the bit inverse of the version for the base station 3, and the
synchronisation word recognisers 137, 139 can be switched between
recognising the handset words and recognising the base station
words by inverting one of the two inputs to each bit comparator, or
by inverting the input to the shift register. Thus, the recognisers
are built so that they can recognise either the handset words or
the base station words, and in use which word is recognised is
determined by a signal on line 141, which indicates whether the S
channel controller has been mounted in a handset 11 or a base
station 3.
The S channel controller 81, 101 also includes a CHM
synchronisation word generator 143 and a SYNC synchronisation word
generator 145. These may each be constructed by providing a
parallel input, serial output 24 bit shift register, with the
parallel inputs hardwired to provide the appropriate
synchronisation word. Each synchronisation word generator 143, 145
is designed to be able to generate either the handset words or the
base station words, by inverting the inputs or the output of the
shift register, and the signal on line 141 determines which words
will be generated in operation of the generators.
As has previously been explained, the S channel controller 81, 101
of a handset 11 will recognise the base station words and generate
the handset words, while the S channel controller 81, 101 of a base
station 3 will recognise the handset words and generate the base
station words. The outputs of the synchronisation word generators
143, 145 are combined by an OR gate 147, and are provided as the S
channel input to the programmable multiplexer 65, 85.
The "CHM recognised" signal and the "SYNC recognised" signal are
provided, when the respective synchronisation word is recognised,
directly to the system controller 79, 99 on respective lines 149,
151, and are also provided to a frame timing controller 153. The
frame timing controller 153 also receives from the system
controller information about which multiplex structure the received
data is assumed to have, and also information about the state of
the radio link. By combining the timing of the "CHM recognised" or
"SYNC recognised" signal with the information about the multiplex
structure, the frame timing controller 153 can generate a frame
clock signal which provides burst timing information to the system
controller 79, 99. Additionally, when the link state information
indicates that the programmable demultiplexer 75, 95 has been
instructed to demultiplex incoming data in accordance with the
multiplex 2 or multiplex 3 data structure at an assumed burst
timing, the frame timing controller also provides a frame lock
signal 157 to the system controller 79, 99, indicating whether the
timing of the received synchronisation word is in accordance with
the assumed burst timing. The frame clock signal 155 and the frame
lock signal 157 are used by the system controller 79, 99 for such
things as controlling the burst timing of the programmable
multiplexer 65, 85 and the programmable demultiplexer 75, 95, as
has already been explained.
The frame timing controller 153 also provides control signals to
the CHM generator 143 and the SYNC generator 145, when either of
these is required to output the respective S channel
synchronisation word to the programmable multiplexer.
Link Initiation Procedure
FIG. 21 shows in flow diagram form the actions taken by the handset
11 and the base station 3 when the base station 3 initiates a link
with the handset 11. FIG. 22 shows the pattern of data burst
transmissions during this process. FIGS. 23 and 24 are
corresponding flow diagrams and data burst sequence diagrams for
the case when a handset 11 initiates a link with a base station
3.
In each of FIGS. 21 and 23, two flow diagrams are shown: one for
the actions taken by the base station 3 and one for the actions
taken by the handset 11. The flow of actions from step to step is
shown in thick lines, and the passage of radio signals between the
devices in various steps is shown in thin lines.
While the handset is turned on, but is not participating in a link,
it performs a channel-scanning loop. In step H1 it selects the next
channel to scan. In step H2 it transmits nothing on the selected
channel, but connects its aerial 25 continuously to the receiver
73. The programmable demultiplexer 75 passes any input data to the
S channel controller 81. If the S channel controller 81 fails to
detect the fixed part channel marker S channel synchronisation word
CHMF within a predetermined period, the handset 11 abandons the
channel in step H3, and returns to step H1 to select the next
channel. If all channels are scanned in turn without CHMF being
detected, the handset 11 may cease operations for a period to
conserve battery power, before scanning the channels again.
While a base station 3 is also not participating in a link, it will
be performing a similar scanning operation, as is shown in FIG. 23.
This scanning operation will be interrupted if the base station 3
receives a signal such as a telephone ringing signal on the
telephone connection 45, indicating that it is required to set up a
link with a handset 11. In this case, the base station will scan
the available radio channels in step B1 to find an empty
channel.
The base station 3 will then begin to transmit signals using
multiplex 2, in step B2. Between multiplex 2 transmission bursts,
the base station 3 will connect its aerial 43 to its receiver 93,
to detect replies from handsets 11 using the multiplex 2 data
structure with the SYNCP S channel synchronisation word.
In its multiplex 2 transmission in step B2, the base station 3 will
transmit D channel data in a predetermined D channel code word
format. A D channel code word will take several multiplex 2 data
bursts to be transmitted. The structure of data transmissions on
the D channel will be described later.
The D channel code word transmitted by the base station 3 includes
a PID field in which a "portable part identification" code is
placed by the base station 3 identifying the specific handset 11 it
wishes to contact. The D channel code word also contains a LID
field, in which the base station 3 places a "link identification"
code. Various different link identification codes may be used under
different circumstances. When the base station 3 is attempting to
set up a link, the code placed in the LID field will be a base
identification code (BID), identifying the base station 3.
Although the base station 3 will only set up a link with one
handset 11 at a time, it may send out a call in step B2 to a
plurality of handsets, and then establish the link with any one of
them. In the multiplex 2 transmissions by the base station 3 in
step B2, the D channel information which the base station 3 wishes
to transmit is repeated continuously. If the base station 3 wishes
to direct its signals to more than one handset 11, then it will
change the PID code in successive transmissions of the D channel
data so as to call each of the handsets in turn.
When the channel selected by the handset 11 in step H1 is the same
channel as was selected by the base station 3 in step B1, the
handset 11 will detect in step H2 the multiplex 2 transmissions by
the base station 3 in step B2. Therefore, the handset 11 will find
a CHMF code, and will move to H4. In this step, the handset 11 uses
the received CHMF code to achieve burst synchronisation with the
base station 3, and the system controller 79 instructs the
programmable demultiplexer 75 to treat received data as being in
multiplex 2. Accordingly, the multiplex 2 transmissions from the
base station 3 are decoded and the D channel data is passed to the
system controller 79.
The system controller 79 assembles the D channel code words being
transmitted by the base station 3, and examines the PID and LID
fields. If the system controller 79 does not detect its own PID
code within a time-out period, then in step H5 the handset 11 will
conclude that the received call from the base station 3 is not
intended for it, and it will return to step H1. The system
controller 79 may also determine when the sequence of PID codes,
transmitted by the base station 3, is repeated. When this happens,
the system controller 79 should have decoded every PID code being
transmitted, and so if its own PID code has not been detected by
this time, the handset 11 may return to step H1 from step H5 even
if the time-out period has not expired.
If, in step H5, the handset 11 decides to respond to the call from
the base station 3, because it has recognised its own PID code, it
moves to step H6. In this step, it begins to transmit in multiplex
2 as well as listen for multiplex 2 transmissions from the base
station 3. The handset 11 will place the SYNCP synchronisation word
in the S channel, since it is responding to the base station 3
rather than initiating its own call. It will use the D channel to
transmit a reply code word, in which it will place its own
identification code in the PID field, and will place in the LID
field the same code as was received in that field in the
transmissions from the base station 3.
To avoid interference between two or more handsets 11 transmitting
on the same channel simultaneously, in response to a series of call
signals from the base station 3 identifying several handsets 11,
the handset 11 will transmit its reply immediately after it has
received a D channel message containing its own PID code, and not
after receiving a D channel message containing another PID
code.
If the base station 3 detects a reply to its transmissions in step
B2, it will check that the received S channel synchronisation word
is SYNCP, and decode the received D channel information to check
that it recognises the returned PID code, and that the returned LID
code is the same as it sent out. If the base station 3 does not
receive a satisfactory reply within a predetermined period, it
abandons its attempt to establish a radio link on that channel in
step B3. The base station then passes to step B4, where it
determines whether it has been trying to establish this radio link
for more than a time-out period. If the time-out period has not
expired, it will return to step B1, select another free channel,
and make a fresh attempt to establish a radio link. If the time-out
period in step B4 has expired, the base station 3 moves to step B5,
and ceases all attempts to establish a link.
If the base station determines in step B3 that a satisfactory reply
has been received, it proceeds to step B6. It continues to transmit
its call to the handset or handsets, using multiplex 2. If
satisfactory replies are received from all the hand sets which are
being called, the base station 3 replaces CHMF in the S channel
with SYNCF, to avoid alerting any further handsets 11
unnecessarily.
Once a handset 11 has identified that a base station 3 is
transmitting a call to it, it can take action either to accept the
call or to decline it. It may decline it either in response to some
action by the user or it may have been pre-set to decline calls,
for example through a function similar to the known "do not
disturb" function on a conventional telephone. A call will not be
accepted until the user presses one of the link control keys 33 on
the handset keypad 31. Accordingly, in step H7 the handset 11
determines whether any action is required. If no action is needed,
it passes to step H8 at which it determines whether the base
station 3 is still transmitting the call. The base station 3 may
cease to transmit the call either by entering a link with another
of the handsets 11, and changing the transmitted multiplex 2
message accordingly, or by ceasing to transmit on the channel
because no handset 11 has accepted the call within a pre-set
period. If the call is no longer being transmitted, the handset 11
returns to step H1 and once again scans the channels for a fresh
call to it.
If it is determined in step H7 that action is required, the handset
passes to step H9 to determine what the required action is. If the
required action is to refuse the call, it passes to step H10.
In step H10 the handset 11 continues to transmit in multiplex 2,
but changes the code in the LID field to a special "link decline"
code. It continues to transmit its own identity code in the PID
field of the D channel. If the base station 3 receives a "link
decline" message in step B6, it may remove the associated PID code
from the list of PID codes which are being called in rotation by
the base station 3. The handset 11 remains in step H10, and
transmits the "link decline" code in response to detecting its PID
until a time-out period of e.g. 1 second has passed without its own
PID being received. This confirms to the handset 11 that the base
station 3 has received the "link decline" message and has stopped
transmitting this PID code. The handset 11 then returns to step H1,
and resumes scanning channels for further messages indicating that
a base station 3 is attempting to set up a link. Because the base
station 3 with which it was previously in communication is no
longer transmitting the PID code of this particular handset 11, the
hand set will not respond again to the base station 3 even when it
scans the channel being used by the base station, as it will
determine in step H5 that its PID is not being transmitted.
If the handset determines in step H9 that the required action is to
accept the call, it passes to step H11. In this step, it continues
to transmit in multiplex 2, sending the same PID and LID codes in
the D channel as in step H6. However, instead of transmitting its
normal handshake code, it transmits a special handshake code
indicating "link request". It continues to decode the multiplex 2
transmissions from the base station 3, in order to receive the
reply from the base station 3 to the link request.
In step B7, the base station 3 determines whether it has received a
link request message from any of the handsets 11 it has been
calling. If no link request is received within a pre-set period,
the base station 3 passes to step B5, and abandons the attempt to
set up a link. If a link request is received, the base station 3
moves to step B8.
Since the base station 3 is now entering a link, it will change the
S channel synchronisation word in its multiplex 2 transmissions
from CHMF to SYNCF, if this has not already been done in step B6.
It will transmit a reply to the handset 11 in which it replaces its
normal handshake code in the D channel with a "link grant" code. In
the PID field of the D channel code word, the base station 3 will
transmit the identification code for the handset 11 to which it is
granting the link.
In the LID field, it will transmit a different link identity code
from the code transmitted in steps B2 and B6. The new LID code is
an arbitrarily chosen code which identifies this specific link
between the base station 3 and the handset 11. If it ever becomes
necessary to re-establish the link, as described below, the handset
11 will transmit link re-establishment messages using the new LID
code. This enables the handset transmissions under these
circumstances to be identified as an attempt at link
re-establishment, and distinguished from a call from the handset to
set up a new link. If the original base identification code was
used as the LID code throughout an established link, this would
increase the possibility that a link re-establishment message from
a handset 11 would be misinterpreted by a base station 3 as a call
to set up a new link.
When the handset 11 in step H11 receives the link grant message
from the base station 3, it passes to step H12. It will stop
transmitting the link request, and will change the code transmitted
in the LID field of the D channel to the new code sent by the base
station 3.
Once the base station 3, in step B8, has received a transmission
from the handset 3 returning the new LID code, it knows that the
link grant message has been received. Accordingly, the base station
3 moves to step B9. Once the handset 11 has reached step H12 and
the base station 3 has reached B9, the link between them is
established, and they communicate with each other in multiplex 2.
Subsequently, the base station 3 will instruct multiplex 1
communication to begin. In response, the handset 11 moves to step
H13. Once the base station 3 receives multiplex 1 transmissions
from the handset 11, it moves to step B10. B channel transmission
may now begin.
The multiplex 2 transmissions between the handset 11 and the base
station 3 include codes indicating whether each side can support
multiplex 1.4, and following this exchange the two parts agree on
whether to use multiplex 1.2 or multiplex 1.4 before moving to
steps H13 and B10.
FIG. 22 shows schematically the exchange of signals between the
handset 11 and the base station 3 when the base station 3 sends out
a call which is accepted by the handset 11.
First, the base station 3 uses multiplex 2 to transmit a D channel
message 159, sending a call to a first handset 11. Next, the base
station 3 sends out a D channel message using multiplex 2 providing
further D channel information, which any receiving handset may use.
This may include data to be displayed on the display 35 of the
handset to provide the user with information about the call, or may
include a D channel instruction to the handsets to provide a call
signal to the user corresponding to the normal ringing of the
telephone. Next, the base station 3 sends out a D channel code word
163 using multiplex 2, calling a second handset. It then repeats
the D channel message 161. The base station continues to alternate
between calling handsets and sending the general D channel message
161, calling each handset of a group of handsets in turn.
At some point the first handset 11 receives these messages from the
base station 3. Following the next transmission of the D channel
word 159 calling the first handset, it replies by sending a D
channel word 165.
The base station 3 continues to send out the D channel words 159,
163, calling all the handsets in turn, interleaved with the D
channel message 161, and the first handset continues to send its
reply message 165 in response to receiving each call message 159
directed to it, until the handset user indicates that the call
should be accepted. Following the next transmission of the D
channel word 159 calling the first handset, the handset 11 sends a
link request message 167. The base station 3 replies with a link
grant message 169, and the link is established.
The base station 3 and the handset 11 then exchange D channel words
171 using multiplex 2, until the base station 3 instructs the
change to multiplex 1. They then exchange multiplex 1 transmissions
173, carrying the B channel and the telephone conversation
begins.
FIG. 23 is a flow diagram corresponding to FIG. 21, but showing the
actions taken by a handset 11 and a base station 3 when a link is
set up in response to a call made by the handset 11.
If a base station 3 is active but not participating in a link, it
will scan the channels to discover whether any handset 11 is
attempting to call it. In step B21, it will select a channel, and
then in step B22 it will listen for any transmissions on the
selected channel. In step B22 the base station 23 will transmit
nothing. However as described with reference to FIGS. 6 to 9, it
might not listen on the selected channel continuously but might
listen only during every other 1 ms period, synchronised to the
burst timing of an associated base station 3.
During the listen periods in step B22, the programmable
demultiplexer 95 will pass all received data to the S channel
controller 101, in order to detect any CHMP channel marker
synchronisation word transmitted by a handset 11. The base station
3 will only respond to the CHMP synchronisation word, and not the
SYNCP synchronisation word, since reception of the SYNCP
synchronisation word indicates transmission from a handset 11 which
has already made contact with some other base station 3.
If the base station 3 determines in step B23 that the channel
marker code word CHMP has not been received within a predetermined
period, it returns to step B21, selects the next channel, and
begins to listen on that channel. Once the base station 3 has
scanned all the channels, it may turn off for a while to save
power, but this is less important for the base station 3 than for
the handset 11, as the base station 3 will normally be connected to
mains electric power.
If the handset 11 is turned on but is not participating in any
link, it will be performing a similar channel-scanning loop, as
already described with reference to FIG. 21. However, this
operation is interrupted if the user presses a key 33, indicating
that a link to a base station should be established. In this case,
the handset scans the channels in step H21, to select an empty
channel.
In step H22, the handset 11 will begin transmitting on the channel
it has selected using multiplex 3. In between the multiplex 3
transmissions, its programmable demultiplexer 75 will pass any
received data to the S channel controller 81, in order to recognise
the SYNCF synchronisation word which should be contained in any
reply from a base station 3.
In the D channel of its multiplex 3 transmissions, the handset 11
will send a D channel code word having a PID field and a LID field.
In the PID field it will place its own handset identification code.
In the LID field, it may place one of a variety of codes, depending
on the service required by the user.
If the handset is being used as an extension of a domestic
telephone or as a numbered extension of a private branch exchange,
the handset 11 will transmit a LID code indicating that it wishes
to make contact with the specific domestic telephone or private
exchange system with which it has been registered. If the handset
11 is being used with a public "telepoint" system (which is a
system in which a user can make telephone calls through any one of
various base stations in various geographical locations) the LID
code may identify the telepoint company or system with which the
handset is registered and through which the user wishes to make the
telephone call.
In an environment where several competing telepoint systems are
present, it is preferable to define one or more LID codes which the
handset 11 can transmit to make contact with any base station
within range, regardless of the system to which it belongs, and
further LID codes which the handset 11 can transmit in order to
make contact only with base stations of one specified system. A
further, special, LID code may be used to enable a handset 11 to
make contact with a base station 3 purely for the purposes of
registration, so that the base station 3 may receive and store the
PID code of the handset 11, enabling it to be called by the base
station 3 in call set up sequences of the type illustrated in FIG.
21. The handset 11 may also acquire further LID codes from a base
station 3 in such a registration radio link.
The various LID codes are stored in the system controller 79 of the
handset 11 together with its PID code. These codes may be placed in
one of the memories of the system controller 79 during manufacture
of the handset 11 or may be entered subsequently through the keypad
31 in a registration process, or may be received in a registration
radio link as mentioned above.
If the base station 3 determines in step B23 that the handset
channel marker CHMP has been received, it will pass to step B24. In
this step, it will still not transmit, but its programmable
demultiplexer 95 will be instructed to decode received data using
the multiplex 3 structure, having the burst timing derived from the
received CHMP word. Accordingly, the D channel data transmitted by
the handset 11 will now be passed to the system controller 99 of
the base station 3, where it will be decoded. The system controller
99 will examine the PID and LID codes, and decide on the basis of
these whether to respond to the handset 11.
If it is determined in step B25 that no PID and LID codes requiring
a response have been received within a pre-set period, the base
station 3 will return to step B21, select a new channel, and begin
listening for further transmissions from handsets wishing to set up
a link.
If the base station 3 determines in step B25 that it should respond
to the handset 11, it will move to step B26, and begin to transmit
in multiplex 2 with SYNCF in the S channel. The base station 3 will
transmit a D channel data word containing the PID code received
from the handset 11, and an arbitrary LID code to identify the link
being set up. The base station 3 will expect the handset 11 now to
switch to multiplex 2 transmissions, using the SYNCP S channel
synchronisation word.
In the D channel code word transmitted in multiplex 3 by the
handset 11 in step H22, the normal handshake code will be replaced
by the "link request" code. In the D channel code word transmitted
in multiplex 2 by the base station 3 in step B26, in reply to the
hand set 11, the normal handshake code will be replaced by the
"link grant" code.
In step H23, the handset 11 determines whether it has received the
SYNCF synchronisation word from a base station 3 within a pre-set
period. If not, it passes to step H24. In this step, it determines
whether a time out period has expired since the handset 11 first
began to request the link. If the time out period has not expired,
the handset 11 returns to step H21, selects another free channel,
and attempts to establish the link on that channel. If the time out
period has expired, the handset passes to step H25, and abandons
the attempt to set up the link.
If it is determined in step H23 that SYNCF has been received, the
handset 11 passes to step H26. In this step, it temporarily ceases
transmission, and decodes the received multiplex 2 transmissions
from the base station 3, while using the received SYNCF
synchronisation word to achieve burst synchronisation with the
transmissions from the base station 3.
The handset 11 is now able to decode the D channel information
transmitted by the base station 3. In step H27 it determines
whether it has received within a pre-set period a D channel code
word containing its PID and the "link grant" code. If such a D
channel code word is not received within the pre-set period, the
handset 11 moves to step H25, and abandons the attempt to set up
the link. If the handset 11 does receive a link grant message
accompanied by its own PID, it moves to step H28. In this step, it
begins multiplex 2 transmissions, using SYNCP as the S channel
synchronisation word. In its D channel message, it will continue to
send its own PID code, but will change the LID code to the link
identification code received from the base station 3. In this step,
the handset 11 will continue to listen for multiplex 2
transmissions from the base station 3, and will maintain burst
synchronisation with the base station 3.
Once the base station 3 has received a multiplex 2 transmission
from the handset 11, using SYNCP as the S channel synchronisation
word and returning the LID code sent out by the base station 3, it
knows that the link grant message has been received.
The base station 3 now moves to step B27, in which it ceases to
transmit the link grant message, and exchanges D channel
information with the handset 11 using the multiplex 2 data
structure. Once the hand set 11 has reached step H28 and the base
station 3 has reached step B27, the radio link has been
established. Subsequently, the base station 3 will instruct the
beginning of multiplex 1 communication. The handset 11 will move to
step H29 and the base station 3 will move to step B28. B channel
communication may now begin.
FIG. 24 shows schematically the pattern of signals transmitted when
a handset 11 successfully initiates a link with a base station 3.
First, the handset 11 transmits a series of link request messages
175, in multiplex 3. When the base station 3 receives these
messages, and decides to grant the link, it replies with link grant
messages 177 in multiplex 2. On receipt of the link grant messages
177, the handset 11 ceases multiplex 3 transmission, synchronises
its burst timing with the signal from the base station 3, and
begins to decode the received multiplex 2 bursts. When it has
decoded a link grant message 177, the handset 11 begins
transmitting messages 179 using multiplex 2. The two parts continue
to exchange messages 179 in multiplex 2, until the base station 3
instructs the change to multiplex 1 messages 181.
As explained above, in some circumstances the handset 11 may
transmit a link request message in multiplex 3 using a LID code
identifying several base stations 3, any of which the handset 11
can establish the link with. If more than one such base station 3
is within range of the handset 11, the base station 3 which first
scans the channel on which the handset 11 is transmitting will
normally be the first to grant the link, and the link will
successfully be set up with that base station. However, two base
stations 3 may, by chance, transmit link grant messages to a
handset 11 simultaneously. In this case, the handset 11 will most
probably fail to decode either message successfully. Accordingly,
in step H23 the handset 11 will determine that it has not received
SYNCF, and will pass through step H24 to step H21. It will select
another empty channel and repeat its multiplex 3 transmissions on
that channel. Since the handset 11 will not reply to the link grant
messages from the base stations 3 in this case, both base stations
will conclude that the link has failed, and will return to step
B21. Each base station will select a new channel and begin
listening for the transmission of CHMP from a handset 11.
In this case, if both base stations 2 select the next channel, they
will continue to scan each channel at the same time as each other,
and attempts to grant links to handsets 11 will continue to fail
for the same reason. In order to prevent this, the channel selected
in step B21 under these circumstances is not the next channel.
Instead, the base stations 3 follow rules designed to reduce the
probability that they will continue to scan the same channel at the
same time. This may be done by providing a channel selection
algorithm which operates randomly, so that the channel selected in
step B21 under the circumstances is chosen randomly. Alternatively,
each base station 3 may be programmed to return to a particular
channel under these circumstances, and nearby stations are
programmed to return to different channels. The random channel
selection algorithm is preferred. If the base stations are
programmed to return to a specific channel, there is the
possibility that incorrect programming may direct two nearby base
stations to return to the same channel each time, in which case
conflict between them will not be resolved.
D Channel Structure
As mentioned above, messages in the D channel are transmitted using
code words. Each code word is 64 bits long. A string of code words
may be sent in succession, as a D channel data packet. In this
case, the first code word must have a first particular format, and
is known as an address code word (ACW), and the remaining code
words of the packet must have a different specified format and are
known as data code words (DCW). In a packet, an address code word
may be followed by up to five data code words. When only one code
word is sent, not followed by any others, it must be an address
code word.
The rate at which D channel bits are sent is determined by the
multiplex data structure being used in the radio link. It will
always take several bursts to send a single D channel code word.
The maximum speed is provided by multiplex 2, in which 32 bits of
the D channel are sent in each burst, so that two bursts are
required for a code word. The slowest transmission of D channel is
with multiplex 1.2, in which only two bits of D channel are sent
per burst. In this case, 32 bursts are required to carry a code
word.
If at any time, there is no D channel information to be sent, the
multiplex structures nevertheless require D channel bits to be
transmitted. In this case, a signal called "IDLE D" may be sent to
fill the D channel. When IDLE D is being sent, the D channel bits
alternate between 1 and 0.
In order to alert the receiving part that useful D channel
information is about to be transmitted, every address code word is
preceded by a standard 16 bit D channel synchronisation pattern
called SYNC D. As well as informing the receiving part that an
address code word follows, the SYNC D pattern ensures that the D
channel decoding operation of the system controller 79, 99 is
synchronised with the boundaries of the code words, so that each
code word is decoded correctly.
Thus, a typical sequence of D channel transmissions, assembled from
a plurality of data bursts, might be as shown in FIG. 25. A period
during which IDLE D is transmitted ends with the transmission of
the 16 bit SYNC D pattern. This is immediately followed by a 64 bit
address code word, and then one or more 64 bit data code words.
In multiplex 1, the first bit of the SYNC D pattern must always be
transmitted as the first bit of a multiplex 1 burst. In multiplex
2, the 16 bit SYNC D pattern must always be transmitted as the
final 16 bits of a burst, and in multiplex 3 the first bit of the
16 bit SYNC D pattern must always be the first bit of useful D
channel information transmitted after the initial 6 bit D channel
preamble in each repetition period of the first submultiplex. When
multiplex 2 or 3 is being used, this places the SYNC D pattern as
soon as possible after the S channel synchronisation word, and
therefore maximises the likelihood that the SYNC D pattern will be
detected promptly following burst synchronisation.
FIG. 26 shows the structure of a message in the D channel. An
address code word 183, optionally followed by up to five data code
words 185 transmitted in a continuous string, as shown in the first
line of FIG. 26, forms a D channel packet 187, shown in the second
line of FIG. 26. Several packets 187 may be combined to create a D
channel message of any length, as shown in the bottom line of FIG.
26.
In order to maintain at least a minimum rate at which handshake
signals are exchanged, successive packets of a D channel message
are not necessarily transmitted immediately one after another.
Instead, a special address code word, not followed by any data code
words, may be transmitted between successive packets of a message.
The special address code word carries handshake and identification
signals.
FIG. 27 shows the general format of a D channel code word. The code
word is made up of eight octets, each illustrated as one line in
FIG. 27, and each octet is in turn made up of eight data bits.
When the code word is transmitted over the D channel, bit 1 of
octet 1 is transmitted first. This is the top right hand bit in
FIG. 27. Next, bit 2 of octet 2 is transmitted. This is the bit
second from the right in the top line of FIG. 27. The remaining
bits of octet 1 are then transmitted in order. Then octet 2 is
transmitted in order from bit 1 to bit 8. The remaining octets are
transmitted in order in the same manner, so that the last bit of
the code word to be transmitted is bit 8 of octet 8, which is the
bottom left hand bit in FIG. 27.
Bit 1 of octet 1 is used to indicate the type of code word. For an
address code word, the bit is set to "1". For a data code word, the
bit is set to "0". Bit 2 of octet 1 determines the code word
format. An address code word can either be in fixed format or
variable length format. A fixed format address code word is used
for transmitting handshake and identity messages, and will be
described in more detail with reference to FIG. 28. A fixed format
address code word is not followed by any data code words. In an
address code word, bit 2 of octet 1 is set to "0" to define a fixed
format address code word. Bit 2 of octet 1 is set to "1" to
indicate a variable length format code word. Variable length format
indicates that the length of the packet may vary, i.e. data code
words may be present. Data code words are always in variable length
format, and therefore this bit should always be set to 1 for a data
code word. All normal D channel messages are carried by variable
length format code words. A variable length format address code
word may be followed by up to five data code words, but may also
form a packet without being followed by any data code words.
The significance of the remaining bits of octet 1, and all bits of
octets 2 to 6, depends on whether the code word is a fixed format
address code word, a variable format address code word or a data
code word.
Octets 7 and 8 always carry a check code. The first fifteen bits of
the check code, from bit 1 of octet 7 to bit 7 of octet 8, provide
a cyclic redundancy check (CRC) code. Such codes, and methods of
generating them, are well known. Bit 8 of octet 8 is a parity bit,
which is chosen so as to give the whole 64 bit code word even
parity.
The structure of a fixed format address code word is shown in FIG.
28. Bit 1 of octet 1 is set to "1" to indicate that it is an
address code word, and bit 2 of octet 1 is set to "0" to indicate
that it is a fixed format word. Bits 3 and 4 of octet 1 carry the
hand shake code. Bit 5 of octet 1 encodes the multiplex 1
signalling rate. It is set to "1" to indicate multiplex 1.4, and is
set to "0" to indicate multiplex 1.2.
The remainder of octet 1, and octets 2, 3 and 4 carry the PID code.
Preferably, this is divided into two sections. Octet 4 alone
carries a manufacturer identity code, which can be allocated to a
manufacturer by a regulatory authority. The remainder of the PID
code is allocated by the manufacturer, and indicates one specific
handset 11 produced by the manufacturer. Octets 5 and 6 carry the
LID code, and octets 7 and 8 carry the cyclic redundancy check and
the parity bit.
The fixed format address code word is transmitted during link set
up, to carry the PID and LID codes and the "link request" and "link
grant" messages, as described with reference to FIGS. 21 to 24.
"Link request" is transmitted by setting the handshake bits 4 and 3
of octet 1 to "00". "Link grant" is transmitted by setting the
handshake bits 4 and 3 of octet 1 to "01".
When the handset 11 transmits "link request", it sets the
signalling rate bit (bit 5 of octet 1) to "1" if the handset can
support multiplex 1.4, and to "0" if it can only support multiplex
1.2. When the base station 3 sends the "link grant" message, it
will set bit 5 of octet 1 to "1" only if the base station 3 can
support multiplex 1.4 and additionally it has received a bit "1" at
this position from the handset 11, indicating tat the handset can
also support multiplex 1.4. If either device can only support
multiplex 1.2, the base station 3 sets bit 5 of octet 1 to "0" in
the "link grant" message, informing the handset 11 that multiplex 1
transmission will take place using multiplex 1.2. This concludes
the "negotiation" operation between the two devices concerning
which version of multiplex 1 to use.
FIG. 29 shows the structure of a variable format address code word.
In this code word, bit 1 of octet 1 is set to "1" to indicate that
it is an address code word, and bit 2 of octet 1 is set to "1" to
indicate that it is in variable format. The significance of bits 3,
4 and 5 of octet 1 depends of bit 6 of octet 1. If further code
words follow in the D channel packet, bit 6 of octet 1 is set to
"0". In this case, bits 3, 4 and 5 give in binary the number of
further D channel code words in the packet following the code word
in which these bits appear. Thus, if the packet contains three code
words in total, bits 3, 4 and 5 of octet 1 of the address code word
(which will be the first code word of the packet) will be set to
"2" or "010" in binary, to indicate that two further code words
follow.
If bit 6 of octet 1 is set to "1", this indicates that the code
word is the final code word of the packet. In this case, it is
possible that only some of the data carrying octets of the code
word carry useful data. Therefore, in this case bits 3, 4 and 5 of
octet 1 give the number of data carrying octets of the code word
which carry useful data. When the code word is interpreted by the
receiving system controller 79, 99, it will use this information to
ignore any remaining octets, which do not carry useful data, in the
final code word of a packet.
Bit 7 of octet 1 is set to 1 to indicate that the current packet is
followed by further packets of D channel information, and is set to
"0" for the last packet of a D channel message.
Bit 8 of octet 1 is set to "0" to indicate that octet 2 has its
normal significance as a control message. In the illustrated
embodiment, this is always the value of this bit, but it provides
the facility of redefining the meaning of octet 2 of a variable
format address code word, if this is desired. Through this
redefinition of the meaning of octet 2, this bit enables the
interpretation of the entire packet to be changed.
Octet 2 of the variable format control word is a control octet. Bit
3 of octet 2 is set to "1" to indicate that the receiving device
must acknowledge successful reception of the D channel packet. In
this case, bit 4 of octet 2 will be the packet number, and
alternates between "0" and "1" for successive packets. Bit 2 of
octet 2 is used to acknowledge received D channel packets from the
device at the other end of the link, when this is required. This
bit is set to the expected value of bit 4 of octet 2 of the address
code word for the next packet to be received from the other device.
When bit 3 of octet 2 is set to "0", acknowledgement of the packets
is not required, and bit 4 of octet 2 has no significance. Whether
bit 2 of octet 2 has significance will depend on whether the device
at the other end of the radio link has requested acknowledgement of
its packets.
Bit 3 of octet 2 must be set to "1", requiring that packets are
acknowledged, whenever a D channel message contains more than 1
packet.
Bit 1 of octet 2 is set to "0" if the last D channel packet
received by the transmitting device was accepted. If a received D
channel packet is rejected, e.g. because the CRC check fails for
one of the code words, bit 1 of octet 2 in the next transmitted
variable format address code word is set to "1", and bit 2 of octet
2 is set to the value of bit 4 of octet 2 in the address code word
of the received, but rejected, packet.
Bit 5 of octet 2 specifies whether the D channel packet is
"information type" or "supervisory type". The contents of
"supervisory type" packets (bit 5 is set to "0") relate to
operations controlling and maintaining the radio link. Such packets
may include instructions asking the other device to increase or
decrease the power at which it is transmitting, to re-establish the
link on the same channel, or to re-establish the link on another,
specified, channel. Another supervisory message is the FILL-IN
message, which serves a special purpose which will be described
later.
All other D channel messages are carried by "information type"
packets (bit 5 is set to "1"). These will include messages sent by
a base station 3 to instruct a handset 11 to emit a ringing tone to
alert a user of an incoming call, or to send messages to be
displayed on the display of the handset 11. "Information type"
messages sent by a handset 11 to a base station 3 typically inform
the base station 3 that certain keys of the keypad 31 have been
pressed. The messages for changing multiplex structure between
multiplex 1 and multiplex 2 are also carried by "information type"
packets.
D channel messages are only permitted to be constructed from more
than one packet, as shown in the bottom line of FIG. 26, if the
packets are "information type" packets. "Supervisory type" packets
must each be independent, with bit 7 of octet 1 of the address code
word set to "0".
Octets 3, 4, 5 and 6 of the address code word carry the D channel
message contents. Octets 7 and 8 carry the CRC code and the parity
bit.
FIG. 30 shows the structure of a data code word. Bit 1 of octet 1
is set to "0", to indicate that it is a data code word, and bit 2
of octet 1 is set to "1", as data code words are only permitted in
variable format. Bits 3, 4, 5 and 6 of octet 1 have the same
meaning as for the variable format address code word shown in FIG.
29. Bits 7 and 8 of octet 1 have no significance, and are set to
"0".
The data code word does not include a control octet, and
accordingly the D channel message contents are carried by octet 2,
octet 3, octet 4, octet 5 and octet 6. Octets 7 and 8 carry the CRC
code and the parity bit.
In "information type" packets, the D channel messages in the
message content portions of the code words are provided in
"identifier, length, contents" format, in a fashion similar to that
already known for ISDN data. In this format, bit 8 of the first
octet of the message is set to "1" to indicate a fixed length
message, and is set to "0" to indicate a variable length message. A
fixed length message consists of only one octet. Unless it is known
that a packet continues a message started in a previous packet, it
is always assumed that the first message contents octet of the
address code word in an "information type" packet (i.e. octet 3 of
the address code word) is the first octet of a D channel
message.
FIG. 31 shows the format of a fixed length message. Bit 8 is set to
"1" to identify that it is a fixed length message. Bits 7, 6 and 5
provide a code identifying the type of message being carried. Since
the message is in fixed length format, no length information is
required, and bits 4, 3, 2 and 1 provide the message contents.
This message format is used only to carry very simple messages,
such as messages controlling the manner in which variable length
format messages should be interpreted.
FIG. 32 shows the format of a variable length D channel message.
This consists of at least three octets, and maybe more.
In the variable length format, bit 8 of the first octet is set to
"0", to indicate that it is a variable length format message. The
other seven bits of the first octet provides the identifier code,
identifying the type of message being sent. The second octet is the
length code. This is the number of remaining octets in the message,
following this length code octet. Thus, if the total message is
four octets long, an identifier and format type octet, a length
code octet and two further octets, the length code octet will
indicate that two further octets follow. All remaining octets of
the variable length message carry the message contents.
Handshake Codes and Link Re-establishment
As has already been described, the fixed format address code word
of FIG. 28 is used during link set up to carry the "link request"
and "link grant" messages, to carry the negotiations for selecting
between multiplex 1.2 and multiplex 1.4, and to carry the PID and
LID codes. After the link has been established, this D channel code
word is also transmitted from time to time to carry handshake
signals. The PID and LID codes can be used during the link to
confirm that the link continues to be established between the same
two devices.
In order to maintain continuity of the link, handshake words must
be exchanged with at least a certain minimum frequency. The
separation of D channel messages into packets allows this to be
done. Between successive packets of the same message, the fixed
format address code word can be transmitted, to maintain the
handshake rate. This does not disrupt the transmission of D channel
messages, as every variable format address code word indicates
whether further packets (and therefore further variable format
address code words) will follow in the same message. The fixed
format address code word will be recognised as not being a packet
of the message, and the assembly of the D channel message will
resume when the next variable format address code word is
received.
In the fixed format address code word, bits 3 and 4 of octet 1
carry the handshake message. Therefore, four handshake messages are
possible. "00" means "link request". "01" means "link grant". "10"
means "ID OK". "11" means "ID LOST". The use of "link request" and
"link grant" during link set up has already been described with
reference to FIGS. 21 to 24. These handshake messages are only sent
for the purposes described. At all other times, the normal
handshake message in the fixed format address code word is "ID OK".
This code serves as a handshake code, and also confirms to the
receiving device that the transmitting device has received a
handshake code from the receiving device within a pre-set period.
The "ID LOST" code also serves as a handshake code, but indicates
to the receiving device that the transmitting device has not
received a valid handshake code from the transmitting device within
the pre-set period. The use of "ID LOST" enables failure of a link
to be determined promptly, so that it can be re-established with
the minimum delay.
When two parts, a handset 11 and a base station 3, are connected in
a radio link, each part will transmit a handshake code, using the
fixed format address code word, at a rate not greater than once
every 400 ms, and not less than once every second. The timing of
the transmission of a handshake code word is not dependent on the
timing of the reception of a handshake code word from the other
part. If either part determines that it has not received a valid
code word for more than 1 second, it concludes that handshake has
been lost. If either side has not received a valid code word for at
least 3 seconds, the parts are permitted to re-establish the link
on another channel. However, if either part has not received a
valid handshake code for 10 seconds, attempts to re-establish the
link must cease, and the link must be treated as having been
terminated.
The prohibition on re-establishing on another channel after less
than 3 seconds prevents undesirable rapid channel switching, which
might interfere with the operation of other devices trying to use
other channels, and prevents unnecessary channel switching in
response to a brief burst of radio frequency noise or interference.
The requirement to close down the link 10 seconds after the loss of
handshake prevents attempts to re-establish the link from
continuing indefinitely.
If two parts are exchanging handshake signals very rapidly, which
would be possible using multiplex 2, there is a chance that each
side could receive a valid handshake signal once every 3 seconds,
even though the quality of the link was very poor. Under these
circumstances, the parts would be forbidden to change channel
regardless of the poor quality of the link. Therefore, handshake
signals are not transmitted more frequently than once every 400 ms,
even if there is spare D channel capacity to carry more frequent
handshake codes.
Every time a part transmits a handshake code, it resets a transmit
timer. Every time it receives any of the four possible handshake
codes, it resets a receive timer. If the received handshake code is
"ID OK", it also resets a link timer, but the link timer is not
reset if the received handshake code is any of the other handshake
codes.
When the transmit timer indicates that 400 ms have passed since the
last time the part transmitted a handshake code, it prepares to
transmit a handshake code as soon as the structure of data being
transmitted on the D channel permits. Just before it transmits its
handshake code, it checks the receive timer. Provided that the
receive timer indicates that one of the handshake code words has
been received within the past second, the part transmits the "ID
OK" handshake code. Otherwise, it transmits the "ID LOST" handshake
code. If the part subsequently receives any valid handshake code,
it will reset its receive timer, and return to sending "ID OK"
handshake codes.
If no handshake codes are received, or if only handshake codes
other than "ID OK" are received, the link timer is not reset. Once
the link timer indicates that 3 seconds have passed since the last
"ID OK" was received, the part will automatically begin link
re-establishment. If the part is a handset 11, it will begin to
transmit in multiplex 3, and if the part is a base station 3 it
will begin listening for the CHMP S channel synchronisation word
transmitted in multiplex 3 by the handset 11. Link re-establishment
follows the same procedure as the procedure described with
reference to FIGS. 23 and 24 used to set up a link when the call is
initiated by the handset, except that in the multiplex 3
transmissions by the handset, the code transmitted in the LID field
of the D channel is the most recent link identification code which
was used by the parts in the link which they are trying to
re-establish, and not the code normally used by the handset to
establish a new link.
When the link is re-established, reception of the "ID OK" handshake
code will reset the link timer. If the link timer shows that this
code has not been received within 10 seconds of the last time it
was received, the part will abandon attempts to re-establish the
link.
Because each part does not reset its link timer when the "ID LOST"
handshake code is received, the link timers of the two parts will
always show times within 1 second of each other, and normally less
than 500 ms of each other. This ensures that the two parts begin to
attempt link re-establishment, and if necessary abandon attempts at
link re-establishment, at almost the same time, even if the nature
of the problem with the link is such that signals in one direction
continue to be received successfully while signals in the other
direction are not.
The effect of the "ID LOST" handshake code on the link timers, when
a link breaks down in one direction only, is illustrated in FIGS.
33 and 34. FIG. 33 illustrates the case where signals from the
handset 11 fail to reach the base station 3, although signals from
the base station 3 continue to reach the handset 11.
Initially in FIG. 33, the quality of the link is good and "ID OK"
handshake codes are transmitted by both parts. However,
interference then prevents the base station 3 from receiving
handshake codes from the handset 11, and the last handshake code
received by the base station 3 occurs at time A. At time B, the
base station 3 transmits its next handshake code. As this is less
than 1 second from time A, it transmits "ID OK". However, next time
it transmits a handshake code, it transmits "ID LOST", as it is now
more than 1 second since time A. Additionally, since no further
handshake codes are received by the base station 3, its link timer
is not reset after time A.
Since the handset 11 is continuing to receive valid handshake codes
from the base station 3 at less than 1 second intervals, it
continues to transmit "ID OK" as its handshake code. However, since
it is receiving "ID LOST" instead of "ID OK", the handset 11 does
not reset its link timer after time B.
At time C, which is 3 seconds from time A, the base station 3
prepares for link re-establishment. It stops transmitting over the
radio frequency channel, and begins to scan for multiplex 3
transmissions from the handset 11. At time D, which is 3 seconds
after time B, the handset 11 stops its previous transmissions over
the link, and begins transmitting in multiplex 3, over the same or
different channel, to initiate link re-establishment.
The period between times C and D is the same as the period between
times A and B. Since "ID OK" was transmitted by the base station 3
at time B because this time was less than 1 second after time A, it
can be guaranteed that these two times are less than 1 second
apart. Typically, they will be less than half a second apart.
Therefore times C and D, when the respective parts move to link
re-establishment, will be similarly close together.
FIG. 34 shows the case where, due to interference, the handset 11
ceases to receive handshake signals from the base station 3, but
the base station 3 continues to receive handshake signals from the
handset 11. Initially, the link quality is good, and the parts both
transmit "ID OK" handshake codes. Subsequently, the handset 11
ceases to receive the handshake codes transmitted by the base
station 3, and the last handshake code received by the handset 11
is sent at time E. From this time onwards, the handset 11 receives
no handshake codes at all, and so it does not reset either its
receive or its link timers.
The next time the handset 11 sends a handshake code, it is still
less than 1 second since it received the "ID OK" code at time E.
Therefore the base station 11 transmits an "ID OK" code, at time F.
However, when the handset 11 comes again to transmit a handshake
code, it is more than 1 second after time E, and accordingly it
transmits "ID LOST".
Since the base station 3 continues to receive handshake codes from
the handset 11, it continues to reset its receive timer and
transmits "ID OK".
However, it is now receiving "ID LOST" signals. The last "ID OK"
signal received by the base station 3 is sent at time F, and this
is the last time that the link timer at the base station 3 is
reset.
At time G, the handset 11 is informed by its link timer that it is
3 seconds from time E. The handset 11 therefore stops its previous
transmission of signals over the radio link, and begins to try to
re-establish a link by transmitting in multiplex 3. Shortly
afterwards, at time H, the link timer at the base station 3 informs
the base station that it is 3 seconds after time F, and the base
station 3 also moves to link re-establishment, and begins to scan
for the multiplex 3 transmissions by the handset 11.
Since times E and F must be less than 1 second apart, times G and H
are also less than 1 second apart.
Regardless of the direction in which the link breaks down, it is
always re-established from the handset 11 to the base station 3,
and not vice versa. In some situations, the link may be established
between the handset 11 and any one of several base stations 3 at
different locations. This may be the case if the base stations 3
are part of a public telepoint system, and may also be the case if
the base stations 3 are all connected to the same private branch
exchange, e.g. for a large industrial site where base stations at
various different locations are required to cover the whole area of
the site.
In one of these situations, the base stations 3 will all be
connected to a central controller, e.g. a computer, and a link may
fail because a handset 11 moves too far away from the base station
3 it was in communication with. The handset 11 may now be in range
of another base station 3 of the same system, so that the link
could be re-established with this other base station 3 instead of
the previous one. Since the telepoint or exchange system cannot
track the movement of the handset, it does not know which base
station 3 should be used to re-establish the link. Therefore, the
base station 3 cannot begin transmission.
When a handset 11 begins multiplex 3 transmissions to re-establish
a link, these will be received by any base station 3 within range.
The base station 3 decodes the PID and the LID, and passes these to
the central controller. This is able to recognise from the PID and
LID that the handset 11 is trying to re-establish a link it had
previously established with a different base station 3. The central
controller can then instruct the base station 3 now receiving
signals from the handset 11 to grant the link, and re-connect the
handset 11 to the destination with which it was previously in
communication over a link with the other base station 3.
In cases such as small area intercoms and telephone extensions
where the handset 11 is only ever in communication with one
particular base station 3, it is possible for the base station 3 to
transmit the first radio signals to initiate link re-establishment,
since there is no need to decide which of several base stations 3
should transmit the signals. However, even in this case there is a
benefit in requiring the first radio signals to be transmitted by
the handset 11 and not the base station 3.
First, the base station 3 will typically be a more powerful
transmitter than the handset 11, and if the base station 3
transmits the first radio signals these may be received by the
handset 11 but the reply from the handset 11 may not be received by
the base station 3. Both the handset 1 and the base station 3 will
then be active in attempting to re-establish the link in
circumstances which prevent re-establishment from actually taking
place. If the base station does not transmit until it has received
a signal from the handset, it is more likely that the signal
strengths in both directions are adequate for link
re-establishment.
Second, if the base station 3 transmits the first signals, it will
have to use CHMF, and all idle handsets 11 within range will have
to synchronise and decode the transmissions before discovering from
the PID whether the signals are intended for the particular handset
11 concerned. If the first signals in link re-establishment are
transmitted by the handset 11, using CHMP, and the base station 3
replies using SYNCF, no other handsets 11 will react to the
signals.
Link Quality Checking
The only encoding in the B channel takes place in the encoders 63,
83 of the handset 11 and the base 3. As described above, the
encoders 63, 83 use an adaptive differential pulse code modulation
algorithm to perform data compression. They may also reverse the
values of selected bits of the B channel data according to a
predetermined pattern (which will be reversed by the decoders 77,
97), in order to maximise the number of bit reversals in the serial
data string. However, the B channel will typically not include any
error detecting or correcting codes. In particular, error detecting
or correcting codes require the transmission of code bits, reducing
the number of transmitted data bits available to carry information.
The B channel has an average transmitted bit rate of 32 kbit per
second in each direction, and it is preferable to use all of these
bits for speech information in order to maximise the quality of the
transmitted speech.
Therefore, if there are errors in the B channel, the system cannot
detect this fact directly. However, all multiplex data structures
carry D channel bits, and errors in the D channel can be detected
using the CRC code of the D channel code words. Therefore, the
presence of errors in the B channel during multiplex 1
transmissions can be inferred from the detection of errors in the D
channel.
Typically, signal errors arise in two ways. First, noise,
interference and other problems with the radio link and the
transmitting and receiving systems may cause random errors at any
average bit error rate. Since these errors are random, each bit
position of a multiplex structure is as likely to suffer such an
error as any other. Second, errors may arise from the
misinterpretation of the received signal if the parts of the radio
link lose bit or burst synchronisation. Although all bits of a
burst are prone to errors due to loss of synchronisation, the first
and the last bits of each burst are especially vulnerable. For this
reason, the D channel bits in multiplex 1.2 and multiplex 1.4 are
placed at either end of the data burst, sandwiching the B channel
bits. This ensures that the D channel, in which errors can be
detected, is preferentially vulnerable to errors as compared with
the B channel, in which errors cannot be detected.
Individual CRC failures in D channel code words are used by the
system controller 79, 99 to detect D channel errors so that it can
avoid acting on false D channel messages. This may lead to the
rejection of D channel packets and requests for retransmission,
using the control octet of the variable format address code word as
described with reference to FIG. 29. Additionally, the system
controller 79, 99 uses the pattern in which D channel CRC failures
accumulate over time to provide a measure of the quality of the
radio link, and if the quality fails to meet a pre-set criterion,
either side can initiate link re-establishment, by sending a
message in the D channel to the other part. In all cases, link
re-establishment is actually carried out by the hand set 11
transmitting in the multiplex 3 data structure.
Serious permanent loss of synchronisation between the parts will
lead to continuous errors in the D channel and the system
controller 79, 99 will rapidly decide that the link quality fails
to meet the criterion, whatever criterion is adopted. Accordingly,
the link quality criterion should be chosen in order to provide
desired performance when radio link problems or slight
synchronisation loss cause only some bits to be received in error
whilst most bits are received correctly.
The effect on the B channel of any average bit error rate can be
simulated, and a subjective decision can be taken on what quality
of received speech is acceptable. The pattern of CRC failures in
the D channel for a given bit error rate can also be simulated, and
the patterns for bit error rates leading to acceptable and
unacceptable speech qualities can be compared. On the basis of this
comparison, a pattern of CRC failures in the D channel can be
selected as the link quality criterion to be used by the system
controller 79, 99 in deciding whether or not to request link
re-establishment. Any type of pattern of CRC errors may be chosen
as the link quality criterion, but it has been found conveniently
simple and effective to specify the criterion as a given number of
successive CRC failures uninterrupted by a D channel code word in
which the CRC check is successful.
Assuming that errors occur in the D channel at random, any given
bit error rate will eventually result in an error pattern failing
to meet the quality criterion, and causing link re-establishment.
Using well known statistical methods, it is possible to calculate
the period, for any given bit error rate, during which there is a
50% probability that an error pattern will occur which fails to
meet the quality criterion.
For an ideal criterion, this period should be very short (e.g. a
fraction of a second) for any bit error rate leading to B channel
speech quality which is judged to be unacceptable, so that the link
is rapidly re-established under these circumstances with the
minimum disruption to the telephone conversation being
conducted.
On the other hand, for bit error rates permitting excellent speech
quality over the B channel, this period should be long compared
with the predicted average length of a call between a handset 11
and a base station 3, so that unnecessary link re-establishment is
unlikely to occur during calls in which the B channel quality is
good. In addition to minimising unnecessary link re-establishments,
this reduces the likelihood that a good quality link will be lost,
since there is always a chance that an attempt at link
re-establishment will result in loss of the link, especially at
busy times when most other available channels are being used for
links between other devices.
For intermediate bit error rates, representing B channel speech
quality which is less than perfect but which is acceptable at least
for short periods, the length of time for a 50% probability that
link re-establishment will be attempted will also be intermediate
the short period for unacceptable quality and the long period for
excellent quality.
FIG. 35 shows a flow diagram of the link quality checking
operations of the system controller 79, 99. In this case, the
quality criterion is that the number of uninterrupted D channel CRC
failures in succession must not reach N.
When a link is first established, the system controller 79, 99 sets
a counter C to 0 in step S1. In step S2 it receives and decodes a D
channel code word. In step S3 it determines whether the check code
and the parity bit of the D channel code word have the correct
values. If the values are correct, the error check succeeds and the
system controller returns to S1. Counter C is set to 0, and the
quality monitoring procedure waits until the next D channel code
word is received and decoded.
If the CRC code or the parity bit indicate the presence of an
error, the check in step S3 fails, and the procedure moves to step
S4. In this step, the value of the counter C is increased by 1.
Next, the value of counter C is tested in step S5. If the value of
C has not yet reached N, the quality monitoring process returns to
step S2, and waits for the next D channel code word to be received
and decoded. In this case, the procedure does not pass through step
S1 in returning to step S2, and therefore the value of C is not
reset to 0. If successive D channel code words contain errors, the
link quality monitoring procedure will pass round the loop made up
by steps S2, S3, S4 and S5, and the value of counter C will
continue to increase. If at any time a D channel code word is
received without errors, the procedure returns to step S1 and
counter C is reset to 0.
After N successive D channel code words have been received all
containing errors, the value of counter C will reach N. This will
be detected by the test of the value of C in step S5, and the
procedure will pass to step S6. In this step, it is determined that
the link has failed to meet the quality criterion, and link
re-establishment is initiated.
If the handset 11 or base station 3 concerned has not received an
"ID OK" handshake code during the previous 3 seconds at the time
when it reaches step S6 in FIG. 35, it is permitted to attempt to
re-establish the link on a different radio channel. Otherwise, the
attempt to re-establish the link must be made on the same radio
channel as was being used previously. However, if the errors have
arisen through loss of synchronisation between the parts, rather
than difficulties with radio transmission and reception,
re-establishing the link on the same channel will normally restore
link quality. Additionally, a condition is applied that link
re-establishment on the same channel as was previously used is not
permitted unless at least 300 ms of transmissions in multiplex 1.4,
or at least 600 ms of transmissions in multiplex 1.2 have taken
place over the link since the link was established or since the
most recent link re-establishment.
In an alternative embodiment, the failure of the D channel CRC and
parity errors to meet a quality criterion is used as an indication
that the B channel quality is unacceptably low, but the action
taken in response to this is not (or not necessarily) to initiate
link re-establishment.
In one alternative, the device (handset 11 or base station 3)
detecting the errors reacts by muting the B channel, so that the
user hears nothing instead of hearing the poor quality B channel,
but link re-establishment is not attempted until there is a loss of
handshake (i.e. "ID OK" has not been received) for three seconds,
as described above.
In another alternative, the device reacts by initiating a change
from multiplex 1 to multiplex 2. Because of the increased amount of
D channel and the presence of the S channel, it is easier to
maintain contact over a low quality link in multiplex 2 than in
multiplex 1.
In both cases, a temporary reduction in link quality will result in
a corresponding temporary cessation of B channel communication, but
the link is maintained and B channel communication may be restored
when the link quality recovers. In both cases, it is possible as a
further option to initiate link re-establishment if it is not
possible to restore B channel communication within a time-out
period.
D Channel Fill In
In the fixed format address code word shown in FIG. 28, the PID and
LID codes may have any values, in accordance with the identity
codes which have been given to particular devices or types of
service. Similarly, the message contents octets of the variable
format address code word of FIG. 29 and the data code word of FIG.
30 may adopt any value, depending of the D channel message being
sent. Therefore, there is the possibility that the contents of a D
channel word may, by chance, resemble the SYNC D pattern. If this
happens, the system controller 79, 99 may believe that it has
received SYNC D when it has really received part of a D channel
code word. Therefore the D channel decoding by the system
controller will not be properly synchronised with the received D
channel data, and the D channel data will be misinterpreted.
In most cases, this error is self-limiting. Every address code word
in the D channel must be immediately preceded by SYNC D. If the
system controller 79, 99 is out of synchronisation with the D
channel, it will probably not find the SYNC D pattern when it
expects this pattern to appear again. When this happens, the system
controller 79, 99 will abandon its incorrect synchronisation with
the D channel, and will search the D channel for the SYNC D
pattern, which will enable it to re-establish correct
synchronisation.
However, a problem can arise if successive address code words in
the D channel contain patterns which resemble SYNC D, at the same
relative positions in the code words. In this case, the system
controller 79,99 can become locked into the incorrect D channel
synchronisation. In order to avoid this, successive address code
words must be spaced by 48 bits of IDLE D, unless it is guaranteed
that the address code words do not contain a pattern resembling
SYNC D or the two successive address code words are sufficiently
different that they cannot carry patterns resembling SYNC D at the
same relative position.
D channel code words must be transmitted with sufficient frequency
to permit satisfactory channel quality monitoring using the CRC
code and the parity bit in each code word. If there are a large
number of D channel messages to be sent, this requirement is met by
the almost continuous stream of code words required to carry the
messages, interleaved with fixed format address code words to carry
handshake signals as required. However, if the same D channel
message is to be sent repeatedly, so that the same variable address
code word is sent repeatedly, or if no D channel messages are to be
sent so that only the fixed format address code word (which will be
the same every time) is to be sent, the address code words must be
spaced by 48 bits of IDLE D to avoid the possibility of locking
onto false D channel synchronisation, as discussed above. IDLE D is
not a D channel code word, but simply a pattern of alternating
"1"and "0" bit values and it does not include a CRC code. In view
of the slow rate at which D channel data is transmitted in
multiplex 1, this requirement to send 48 bits of IDLE D may mean
that the rate at which D channel code words is sent is insufficient
for satisfactory channel quality monitoring.
To solve this problem, a special "FILL-IN" D channel code word is
defined. This code word is a "supervisory type" variable length
format address code word, not followed by any data words and
carrying a special message in all "contents" octets (i.e. octets 3,
4, 5 and 6) which is defined to have no meaning. The FILL-IN code
word is designed so that no part of it, including the check code in
octets 7 and 8, resembles the SYNC D sequence. Therefore, the
FILL-IN word can be sent continuously, to maintain the D channel
error check rate, when there are no D channel messages to be
transmitted. Because it is known not to contain a false
representation of SYNC D, there is no need to precede repeats of
this word by 48 bits of IDLE D. Additionally, if it is desired for
any reason to transmit another address code word repeatedly, the
FILL-IN word may be interleaved with the other address code word,
in place of the 48 bits of IDLE D, to provide a guarantee that the
receiving system controller cannot lock into a false D channel
synchronisation, while at the same time the FILL-IN word maintains
the rate of D channel code words for use in channel quality
monitoring.
FIG. 36 shows the bit pattern of octets 1 to 6 for a D channel word
which is suitable for use in a system with a SYNC D pattern of
"0010001111101011". The "X" given at bits 1, 2 and 4 of octet 2
indicate that these bits may be either "1" or "0". The pattern
"11110000" in octets 3 to 6 is effectively a "supervisory type"
message of no meaning. This same bit pattern is used in the last
code word of an "information type" packet to fill up octets not
used by the message being transmitted.
S Channel Word Structure
The S channel synchronisation words, SYNCP, SYNCF, CHMP and CHMF
are used during multiplex 3 and multiplex 2 transmissions to enable
the receiving device to obtain burst synchronisation with the
transmitting device. Until the relevant synchronisation word has
been detected, the programmable demultiplexer 75,95 cannot achieve
burst synchronisation with the incoming data, and it is not
possible to interpret the D channel.
Since the S channel synchronisation word must be detected before
burst synchronisation can be achieved, it must be possible to
detect the word asynchronously. For this reason, once bit
synchronisation has been achieved, each bit of the incoming data is
passed to the S channel controller 81,101, and in each bit period
the S channel controller compares the pattern of the 24 most
recently received bits (assuming that the S channel synchronisation
words are 24 bits long) with the stored target word patterns. In
order to allow the S channel synchronisation words to be detected
in the presence of a small amount of noise, the S channel
controller provides a "word found" output if the input bits match
the target pattern for at least 22 of the 24 input bits.
In order to avoid the possibility that the receiving device will
incorrectly identify the presence of an S channel synchronisation
word, and therefore obtain incorrect burst synchronisation, it
should ideally not be possible to obtain in multiplex 2 or
multiplex 3 a pattern of data which, if correctly received,
provides a match to 22 of the 24 bits of any synchronisation word,
except when the synchronisation word itself appears in the data and
is compared with the stored synchronisation word in precisely the
correct alignment. Thus, if the received S channel synchronisation
word is compared with the stored version of itself, but misaligned
by one or more bit periods, or if any other part of the multiplex 2
or multiplex 3 transmissions are compared with the stored S channel
synchronisation word, there should be no recognition that the
synchronisation word is present, or else incorrect burst
synchronisation will result.
These requirements may be considered in general terms for a
synchronisation word of length L, used in a system in which
recognition of the presence of the synchronisation word is deemed
to have occurred if a comparison between the bit pattern of the
word and the bit pattern of incoming data gives no more than K
errors for any string of L data bits in the incoming signal. In
this generalised case, the following conditions are each
individually helpful, and are preferably both present.
A) Each data burst has fixed and variable portions, and each
possible string of L consecutive bits contains fewer than L-K
variable bits. It is assumed that the variable portions can assume
any values, and so there is the possibility that by chance L
successive bits of variable data will provide precisely the same
pattern as the S channel synchronisation word. By splitting the
variable data so that L consecutive bits contain less than L-K
variable bits, the possibility is avoided that L-K bits of such a
pattern of variable data may occur as an unbroken bit stream in the
data burst, leading to false recognition of the S channel
synchronisation word.
In the multiplex 2 structure, the D channel portions are variable
portions and the S channel portion (preamble plus S channel
synchronisation word) is a fixed portion. Although the multiplex 2
burst carries 32 bits of D channel data, this is split into two
16-bit portions separated by 34 bits of the S channel, so that the
D channel alone can never mimic 22 or more bits of the 24-bit S
channel synchronisation word. The amount by which the maximum
number of variable bits in any string of L consecutive bits is less
than L-K can be regarded as a protection factor. Thus, if the
number of variable bits is one bit less than L-K bits, it provides
a protection factor of one bit. If it is two bits less, it provides
a protection factor of two bits. In view of the possibility that
errors in the received data may cause the fixed data burst portion
next to the variable data burst portion to mimic the extra bits of
the S channel synchronisation word which cannot be contained in the
variable bits because of their fewer number, a higher protection
factor gives better protection against the possibility that chance
resemblance of variable data to the S channel synchronisation word
may lead to incorrect burst synchronisation. In the case of
multiplex 2, L-K is 22 whereas any string of 24 consecutive bits
can only include a maximum of 16 variable bits, providing a
protection factor of 6 bits.
B) Any string of L bits made up wholly of a fixed data portion of a
burst must give more than K errors when compared with any of the
synchronisation word patterns. This applies to all strings of L
bits of fixed data, including strings containing part of a
synchronisation word itself, except for the string made up
precisely of the correct synchronisation word in its correct
position. Additionally, any string of L bits made partially of
fixed data and partially of variable data, must also give more than
K errors when compared with any of the synchronisation word
patterns, while assuming that the variable data bits give no
errors. Again, it is possible to define a protection factor. In
this case, the protection factor is the number of errors in excess
of K provided on this basis by the string of L bits of fixed data
or part fixed and part variable data which gives the least number
of errors in comparison with any of the synchronisation word
patterns.
Theoretically, it is possible to discover the patterns of L bits
which meet condition B) for any given burst structure or, if there
are no such patterns, to discover the patterns which come closest
to meeting condition B), by comparing all possible patterns of L
bits with the burst structure at all possible bit offset positions.
In practice, for any reasonably large value of L, there are so many
possible L-bit patterns that such a comparison cannot be carried
out in a reasonable amount of time. However, since the
synchronisation word itself forms all or part of the fixed portion
of the data burst, the degree of self-correlation between each
synchronisation word pattern and itself offset by one or more bits,
and the cross-correlation, with or without offset, between
different synchronisation word patterns, are both relevant to
condition B) above.
In the worst case, it can be assumed that the S channel
synchronisation word is embedded in the multiplex structure in
variable data. Under these circumstances, if M is the number of
matches of the synchronisation word with itself offset by S bits,
then for all values of S, M+S must be less than L-K, to meet
condition B) above. The amount of the offset S is added to the
number of matches M, to take account of the possibility that all
bits of the variable data may by chance match precisely with the
bits of the synchronisation word they are compared with. As S
approaches L, so that the synchronisation word is offset to such a
high degree that it only overlaps itself by a very few bits, this
condition becomes harder to meet. However, condition A) above
prevents S from reaching L-K, as it prohibits the presence of this
much variable data in L successive bits.
For any reasonably high value of L (i.e. for any reasonably long
synchronisation word) it can be extremely arduous to find all
possible patterns of length L which meet this condition, or to find
the patterns of length L for which the highest value of M+S remains
below L-K by the greatest amount. Nor is it necessarily appropriate
to do so, as it is possible to use burst structures such as
multiplex 2 and multiplex 3 in which the S channel synchronisation
word is not embedded in variable data. In multiplex 3 the S channel
synchronisation word is provided with 12 bits of preamble on either
side and in multiplex 2 it has 10 bits of preamble in front of it
and the variable data behind it is restricted to 16 bits. As a
practical matter, it is reasonable to assume that bit patterns
having low self-correlation and cross-correlation side lobes will
tend to be suitable for condition B) above, as they will tend to
have low values of M.
The value of a self-correlation side lobe in the comparison of a
pattern with itself at an offset of S bits is defined, for the
purposes of this patent application, as being the number of matches
in the comparison of the pattern with itself at this offset, minus
the number of mismatches. If the value of the self-correlation side
lobe is calculated for all values of S (except S=0: correct
alignment), the maximum of all the values of the self-correlation
side lobes found for all the values of S can be taken as a measure
of the degree of self-correlation of the bit pattern. The lower
this value, the more promising the pattern is as a candidate for an
S channel synchronisation word.
The cross-correlation side lobes are defined in the same way, for
the comparison of one synchronisation word pattern with another,
except that in this case the value at S.ltoreq.0 must also be taken
into consideration.
In the design of the illustrated embodiment, condition A) was met
by the design of the multiplex 2 and multiplex 3 burst structures.
In this case, L, the length of the S channel synchronisation word,
is 24 and K, the number of permitted errors in recognising the S
channel synchronisation word, is 2. In any continuous string of 24
bits transmitted in multiplex 2 or multiplex 3, the maximum number
of D channel bits which can be present is 16, giving a protection
factor of 6 bits for condition A).
It was decided to seek to meet condition B) by appropriate choice
of bit patterns for the S channel synchronisation words. The
preamble bits in the S channel in multiplex 2 and multiplex 3 are
present to enable the receiving device to obtain bit
synchronisation, and it might interfere with this objective if this
bit pattern was altered to improve performance on condition B) with
an arbitrarily chosen S channel synchronisation word pattern. The
preamble bits in the D channel in multiplex 3 are also useful for
enabling bit synchronisation, and additionally the pattern is the
same as the pattern for IDLE D, so that it is unlikely to result in
misinterpretation of D channel data. Therefore, it was also
considered to be undesirable to attempt to manipulate these bit
patterns to improve performance on condition B).
In order to simplify calculations, it was decided first to identify
good candidate patterns for the synchronisation words, by selecting
those with good self-correlation properties, i.e. those with low
values for the highest value self-correlation side lobe. All
possible 24 bit binary patterns were examined to identify those
with good self-correlation properties by this definition.
Because of the large number of calculations involved, the candidate
patterns were not tested to ensure that condition B) was met for
every possible string of 24 bits in multiplex 2 and multiplex 3.
Instead, a multiplex 3 test and an S channel test were used.
In the multiplex 3 test, a 24 bit candidate pattern was compared
with the 8 preamble, 10 data, arrangement of bits used in the D
channel in multiplex 3, at each of the 18 possible different bit
offsets. After 18 bit positions of offset, the arrangement of bits
repeats, so that comparison at further offsets is unnecessary. The
result of the test was the maximum number of matches obtained with
any comparison bit pattern. To satisfy condition B) on this test,
the maximum number of matches had to be less than L-K, i.e. less
than 22. In accordance with condition B), it was assumed that every
bit in the D channel provided a perfect match to the corresponding
bits in the 24 bit candidate pattern.
The candidate pattern is aligned with the maximum number of
variable data bits when it is aligned with one 8 bit preamble
portion and part of each of the 10 bit data portions on either side
of the preamble portion. In this case, the 24 bit pattern is
aligned with 16 variable bits. This is the same as the maximum
number of variable data bits a pattern can be aligned with in
multiplex 2, and it is believed that the separation of the variable
bits in multiplex 3 into two portions of up to 10 bits, rather than
one portion of 16 bits as in multiplex 2, provides a more stringent
test for the 24 bit candidate pattern.
Additionally, in multiplex 2 it takes two bursts to transmit a D
channel code word, and as has been described above with reference
to the FILL-IN word, there are rules which prevent the continuous
unbroken repetition of the same D channel code word. Therefore the
variable data portions of multiplex 2 bursts will be different from
burst to burst, and will not repeat for several bursts. Thus any
variable data pattern in a multiplex 2 burst leading to incorrect
recognition of the S channel synchronisation word will not be
repeated, and a recovery from incorrect recognition to correct
recognition should occur rapidly. In multiplex 3 the variable (D
channel) portions tend to be the same from burst to burst, making
recovery from incorrect recognition of the S channel
synchronisation word more difficult. Therefore the avoidance of
incorrect recognition is more important in multiplex 3 than in
multiplex 2.
For these reasons, it was not considered necessary to carry out a
corresponding test using the multiplex 2 pattern of variable data
bits.
The best self-correlation performance found amongst all possible 24
bit binary patterns provided a value of +1 as the highest
self-correlation side lobe value. Some of these values also met
condition B) in the multiplex 3 test. However, all of these
provided at least one position in the multiplex 3 test where 21
matches were possible. That is to say, these patterns provided a
protection factor for condition B) of only one bit in the multiplex
3 test.
It was considered that this was not satisfactory, as this meant
that a false recognition of an S channel synchronisation word would
be possible with some particular pattern of D channel data in
multiplex 3 if a single error arose in the reception of the data.
Since the D channel data transmitted in multiplex 3 is the PID and
LID codes, this meant that a handset 11 with a particularly
unfortunate PID code might suffer an excessive rate of failure when
attempting to initiate a link, because the base station would be
prone to mis-identify part of the D channel as the S channel
synchronisation word in the presence of a small degree of noise,
and therefore fail to decode the multiplex 3 transmissions
correctly. Therefore all candidate patterns having a maximum
self-correlation side lobe value of +1 were rejected, and candidate
patterns having a maximum self-correlation side lobe value of +2
were considered. Several such codes were found which provided a
protection factor of 2 bits for condition B) in the multiplex 3
test.
By accepting candidate bit patterns with self-correlation side
lobes of up to +2, the chance is increased that incorrect burst
timing will be obtained in a noisy environment, by misinterpreting
S channel data offset from the correct S channel synchronisation
word timing. However, it was considered that this situation was
preferable to the possibility that some particular devices would
suffer the problems which might flow from a protection factor of
only 1 bit in the multiplex 3 test.
Pairs of bit patterns were identified which gave a bit protection
factor of 2 for condition B) in the multiplex 3 test, such that
each member of a pair was the bit inverse of the other. The use of
bit inverse pairs means that it is unnecessary to consider the
effect of the polarity of the D channel and S channel preamble
portions. Seven such codes, fourteen such patterns, were found.
Each of these patterns was compared with each of the others,
including its bit inverse, to determine cross-correlation values.
The 2 bit inverse pairs of patterns having the lowest maximum value
of cross-correlation lobes for the 6 cross-correlations between
them were selected.
The S channel test was carried out on the selected patterns. In
this test, each of the four codes was compared at all offsets with
each of four comparison patterns. The comparison patterns were 36
bits long, and consisted of 12 preamble bits and then a respective
one of the four candidate patterns. The comparison pattern is the
same as the structure of one repeat in the S channel submultiplex
of multiplex 3. It also includes within it the pattern of the S
channel in multiplex 2. Therefore this test provides an indication
of the probability that S channel data in multiplex 2 or multiplex
3 will be incorrectly identified as the wrong S channel
synchronisation word, or as the right word with the wrong
timing.
In the S channel test, when each candidate pattern is compared with
the test pattern containing itself, there will be a complete 24 bit
match when the candidate pattern is aligned with itself in the test
pattern. This data is irrelevant, since it represents a correct
decoding of the S channel, rather than an incorrect decoding, and
therefore it was discarded. After this irrelevant data had been
discarded, the results for each bit inverse pair were studied to
determine the alignment with any of the test patterns giving the
greatest number of matches. For one bit inverse pair, this greatest
number was 15 matches and for the other the greatest number was 14
matches.
Thus, the S channel test gave protection factors of 7 bits and 8
bits respectively for condition B).
The bit inverse pair giving a maximum of 14 matches, and a
protection factor of 8 bits, were selected as the channel marker
codes CHMF and CHMP, and the other pair were selected as the
ordinary S channel synchronisation words SYNCF and SYNCP.
It should be noted that the protection factors for condition B)
given by the multiplex 3 test and the S channel test only apply to
the particular group of alignments between the synchronisation word
and the burst structure for which the tests are carried out, and do
not necessarily guarantee that these protection factors are
provided for all possible alignments between the synchronisation
word and the burst structure.
However, the multiplex 3 test replicates the arrangement of bits
throughout the first four submultiplexes of multiplex 3, and the S
channel test replicates the arrangement of bits in the fifth
submultiplex of multiplex 3. At the transition between the fourth
and fifth submultiplexes, ten bits of variable D channel data are
followed by fourteen preamble bits (two from the D channel in the
fourth submultiplex and twelve from the S channel in the fifth
submultiplex) before the twenty four W bits making up the S channel
synchronisation word. It is valid to treat preamble and W bits
(which are fixed) as particular values for variable bits in order
to fit this arrangement to the arrangement used in the multiplex 3
test. Therefore, the results of the multiplex 3 test and the S
channel test between them provide a guarantee that a protection
factor of at least two bits is available for condition B) at all
possible alignments of the synchronisation word and the multiplex 3
burst structure.
The values chosen for the S channel synchronisation words,
expressed in hexadecimal and binary notation, are as follows:
______________________________________ CHMF: BE4E50 hexadecimal;
101111100100111001010000 binary CHMP: 41B1AF hexadecimal;
010000011011000110101111 binary SYNCF: EB1B05 hexadecimal;
111010110001101100000101 binary SYNCP: 14E4FA hexadecimal;
000101001110010011111010 binary
______________________________________
As will be appreciated by those skilled in the art similar
performance could be obtained with a set of 4 bit patterns for the
S channel synchronisation words which were the same patterns as
given above in reverse bit order. These patterns would be 0A727D,
F58D82, A0D8D7 and 5F2728 in hexadecimal notation.
As a further test for the bit patterns selected as the
synchronisation words, their performance when embedded in variable
data was examined. It was found that for each of the selected bit
patterns, there was at least one number S of bit positions by which
the pattern is offset from its correct position at which M+S is
equal to or greater than L-K. That is to say, if it is assumed that
all of the bits of variable data provide a perfect match, there is
at least one offset value S at which the total number of matches to
the 24 bit synchronisation pattern is at least 22. As mentioned
above, this is in fact inevitable once S reaches 22.
The bit patterns selected for the synchronisation words are
nevertheless suitable in practice. In the multiplex 3 data
structure, the synchronisation word is never embedded in variable
data, but is always preceded by 12 bits of preamble, and is either
followed by 12 bits of preamble or, in the case of the last repeat
in the fifth submultiplex, is followed by the end of transmission.
As noted above, there is a guarantee that false recognition cannot
occur in multiplex 3 unless at least two bits are wrongly received
(e.g. due to noise). In multiplex 2, the S channel synchronisation
word is always preceded by 10 bits of preamble, and is followed by
only 16 bits of variable D channel data before the end of
transmission. Therefore, even allowing for the two permitted errors
in recognition, offset values of greater than 18 for which the
synchronisation word is embedded entirely in variable data, which
lead to the greatest probability of false recognition, cannot
occur.
Furthermore since the 10 preamble bits in multiplex 2 come before
the synchronisation word, and the 16 D channel bits adjacent to the
synchronisation word come after it, the adjacent 16 D channel bits
can only lead to false recognition of the S channel synchronisation
word if the S channel controller 81, 101 has failed to detect the
synchronisation word when it actually occurred. If the S channel
controller 81, 101 recognises the presence of the synchronisations
word when it does occur, the frame timing controller 153 will use
the timing of this recognition to set the frame clock 155, and will
ignore a further recognition signal from the synchronisation word
recognisers 137, 139 should such a further, erroneous, output be
provided a few bit periods later.
Finally, occasional incorrect recognition in multiplex 2 does not
matter, provided that its frequency is low, since recovery from
incorrect recognition tends to happen anyway in multiplex 2 as
explained above.
In order to determine that the likelihood of such an erroneous
recognition was acceptably low, it was assumed that the
synchronisation word was embedded in randomly variable data. In
this case, the probability that false recognition will occur at an
offset of any particular number S of bit periods is the probability
that S bits of random data will provide at least (N-K-M) matches,
where M is a number of matches which the synchronisation word has
with itself at an offset of S bits, N is the total length of the
synchronisation word (i.e. 24), and K is the number of errors
permitted in successful recognition (i.e. 2).
For any given value of S, this probability is: ##EQU1## is a
binominal coefficient.
The sum of these probability values for all values of S, i.e. from
S=1 to S=23, provides a figure for the probability or frequency of
a false detection output if the synchronisation word is embedded in
random data. Cross correlation values, i.e. figures for the false
recognition of one synchronisation word when another is embedded in
random data, may be provided using the same formula, but the
probability for zero offset, i.e. S=0, should also be included.
Tables 1, 2 and 3 below give the peak side lobe value, peak number
of matches and false detection value for each of the
synchronisation words when compared with itself and when compared
with each of the other synchronization words and also the "0101 . .
. " preamble pattern.
TABLE 1 ______________________________________ Side Lobes CHMF
SYNCF CHMP SYNCP 0101 . . . ______________________________________
CHMF 2 10 6 7 4 SYNCF 2 7 6 5 CHMP 2 10 4 SYNCP 2 5
______________________________________
TABLE 2 ______________________________________ Matches CHMF SYNCF
CHMP SYNCP 0101 . . . ______________________________________ CHMF
12 16 11 13 12 SYNCF 11 13 12 12 CHMP 12 16 12 SYNCP 11 12
______________________________________
TABLE 3 ______________________________________ False Detection
Values CHMF SYNCF CHMP SYNCP 0101 . . .
______________________________________ CHMF 2.7E-5 2.89E-4 1.50E-3
4.80E-4 2.77E-4 SYNCF 6.9E-5 4.80E-4 1.42E-3 5.95E-4 CHMP 2.7E-5
2.89E-4 1.40E-4 SYNCP 6.9E-5 3.55E-4
______________________________________
When comparing tables 1 and 2, it should be noted that the peak
side lobe value and the peak number of matches for any comparison
do not necessarily occur at the same amount S of offset. In table 3
"E" stands for "exponential", and means that the first number
should be multiplied by 10 to the power of the second number. Thus
1.42E-3 means 0.00142.
For comparison, it may be noted that the 24 bit pattern
111100001111000011110000 has a peak self correlation side lobe
value of 16, its greatest number of matches at any offset is 18,
and its false detection value is 1.47E-1 or 0.147 (i.e. assuming
that it was embedded in random data, it would cause an incorrect
recognition output for itself having a wrong timing on 14.7 percent
of occasions in which the pattern appeared).
Modifications and Alternatives
FIG. 37 is a schematic diagram, similar to FIG. 1, showing a
telecommunications network 1 having network links 9 to modified
base stations. Base station 189 has a single network link 9 to the
communications network 1, and resembles the base stations 3 of FIG.
1 in this respect. However, it has a distributed antenna 191, e.g.
a "leaky feeder", in place of the conventional aerial 43 of the
base station 3. This may permit improved geographical coverage by
the base station 189, with relatively low power radio
transmissions.
Base station unit 193 has a plurality of network links 9 to the
telecommunications network 1, and can therefore connect a plurality
of handsets 11 to the telecommunications network 1 over respective
network links 9 and respective radio links 13, on different radio
channels. The base station unit 193 may be constructed as shown
schematically in FIG. 38. A plurality of base station control
circuits 55, each similar to the circuit described with respect to
FIG. 16, are connected to respective network links 9 by respective
telephone connections 45. The transmit/receive switches 91 are
connected to a radio signal combiner 195, instead of to respective
aerials 43. Through the action of the combiner 195, each individual
control circuit 55 can transmit and receive using a common aerial
197. In order to prevent the transmissions by one control circuit
55 disrupting receiving operations of another control circuit 55,
the burst timing for all of the base station control circuits 55 of
the base station unit 193 is controlled centrally, so that they
transmit and receive synchronously.
The arrangement of FIG. 39 provides a similar operation to the
arrangement of FIG. 38, but in this case each base station control
circuit 55 has a separate respective aerial 43, in place of the
combiner 195 and common aerial 197. Thus, the arrangement of FIG.
39 closely resembles a collection of base stations 3, each having a
single network link 9, in close proximity. However, in view of the
close proximity of the units, and in particular the close proximity
of their aerials 43, it will normally be necessary to ensure burst
synchronisation between the respective base station control
circuits 55, so that transmissions from one circuit do not swamp an
attempt by another circuit to receive a signal from a handset
11.
FIG. 40 shows a further modification, which permits a base station
unit to connect a plurality of handsets 11 for intercom
communication, or to connect a plurality of handsets 11 to a single
network link 9 to provide a conference call. A plurality of base
station control circuits 199 are provided. In FIG. 40, they are
shown as each having a respective base station aerial 43, as in
FIG. 39, but a combiner 195 and common aerial 197 could be provided
instead. Each base station control circuit 199 is connected to a
switching circuit 201, which is in turn connected to one or more
network links 9 by respective telephone connections 45. The number
of network links 9 to which the switching circuit 201 is connected
may be fewer than the number of base station control circuits 199.
The switching circuit 201 operates under the control of signals
received from the base station control circuits 199, to connect
respective base station control circuits 199 together and/or
connect them to a telephone connection 45, so as to provide
intercom/conference facilities in addition to normal telephone call
facilities.
Each base station control circuit 199 may be the same as the base
station control circuit 55 described with respect to FIG. 16. In
this case, the switching circuit 201 would receive signals from,
and send signals to, the respective line interfaces 103 of the
control circuits. However, it is preferable to provide the base
station control circuits 199 as modifications of the structure
shown in FIG. 16, in which the encoder 83, the decoder 97 and the
line interface 103 are not present. Instead, the switching circuit
201 contains an encoder, a decoder and a line interface for each of
its telephone connections 45.
In this case, the switching circuit 201 receives the B channel data
direct from the programmable demultiplexer 95, and provides this
data to the decoder 97 of the respective telephone connection 45 if
the signals are to be sent over a network link 9, and provides the
B channel signals from the programmable demultiplexer 95 of one
base station control circuit 199 to the programmable multiplexer 85
of another base station control circuit 199 if they are to be
transmitted to a further handset 11. The signals from the system
controller 99, normally sent to the line interface 103, may be used
to control a switching control unit which controls the operations
of the switching circuit 201, or may be passed on to a line
interface 103 associated with a telephone connection 45 or to the
system controller 99 of a further base station control circuit 199,
as appropriate. As in the arrangement of FIGS. 38 and 39, the burst
timing of the operations of the base station control circuits 199
should be synchronised by providing a common burst timing
signal.
In any of the base station arrangements, the aerials 43,197 may be
replaced by a distributed antenna 191 as shown in FIG. 37.
The preferred embodiments of the present invention have been
described largely on the assumption that a radio link is set up
between a base station and a handset in order to permit speech
conversation over the B channel. However, as mentioned with
reference to FIG. 15, the handset 11 may be incorporated in a
personal computer or portable computer terminal, to enable the
radio link to carry computer data signals. In this case, the
computer data signals may be carried by the B channel in multiplex
1, or alternatively the radio link may never move to multiplex 1
and the computer data may be carried as special messages in the D
channel using multiplex 2. Data communications using the B channel
and multiplex 1 will be considerably faster, as each multiplex 1
transmission burst carries 64 bits of B channel, all of which are
available to carry the data. In multiplex 2, only 32 bits of D
channel are carried per burst, and additionally the code word
structure used for carrying D channel messages means that only
about half of the D channel bits are available for carrying the
computer data. However, the use of the D channel to carry computer
data may be advantageous in some circumstances, since the D channel
transmissions are encoded for error detection. If a radio link is
used to communicate computer data via the D channel, the two parts
may communicate only in multiplex 2 once the link has been set up,
and link transmissions may never switch to multiplex 1.
In another modification, a handset 11 may be provided without a
keypad 31, or with only a few keys, so that a telephone number
cannot be dialled from the handset 11. Such a handset may be used
only to receive calls, or may be permitted to make calls only to
one or a few preselected numbers. The numbers may be stored in the
handset and transmitted automatically to the base station.
Alternatively, especially if there is only one number, it may be
stored by the base station and selected in response to the PID of
the handset.
The embodiments described above are provided by way of example, and
various modifications and alternatives will be apparent to those
skilled in the art.
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